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Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
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:title: Dockerfile Reference
:description: Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image.
:keywords: builder, docker, Dockerfile, automation, image creation
.. _dockerbuilder:
====================
Dockerfile Reference
====================
**Docker can act as a builder** and read instructions from a text
``Dockerfile`` to automate the steps you would otherwise take manually
to create an image. Executing ``docker build`` will run your steps and
commit them along the way, giving you a final image.
.. _dockerfile_usage:
Usage
=====
To :ref:`build <cli_build>` an image from a source repository, create
a description file called ``Dockerfile`` at the root of your
repository. This file will describe the steps to assemble the image.
Then call ``docker build`` with the path of your source repository as
argument (for example, ``.``):
``sudo docker build .``
The path to the source repository defines where to find the *context*
of the build. The build is run by the Docker daemon, not by the CLI,
so the whole context must be transferred to the daemon. The Docker CLI
reports "Uploading context" when the context is sent to the daemon.
You can specify a repository and tag at which to save the new image if the
build succeeds:
``sudo docker build -t shykes/myapp .``
The Docker daemon will run your steps one-by-one, committing the
result to a new image if necessary, before finally outputting the
ID of your new image. The Docker daemon will automatically clean
up the context you sent.
Note that each instruction is run independently, and causes a new image
to be created - so ``RUN cd /tmp`` will not have any effect on the next
instructions.
Whenever possible, Docker will re-use the intermediate images,
accelerating ``docker build`` significantly (indicated by ``Using cache``):
.. code-block:: bash
$ docker build -t SvenDowideit/ambassador .
Uploading context 10.24 kB
Uploading context
Step 1 : FROM docker-ut
---> cbba202fe96b
Step 2 : MAINTAINER SvenDowideit@home.org.au
---> Using cache
---> 51182097be13
Step 3 : CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/' | sh && top
---> Using cache
---> 1a5ffc17324d
Successfully built 1a5ffc17324d
When you're done with your build, you're ready to look into
:ref:`image_push`.
.. _dockerfile_format:
Format
======
Here is the format of the Dockerfile:
::
# Comment
INSTRUCTION arguments
The Instruction is not case-sensitive, however convention is for them to be
UPPERCASE in order to distinguish them from arguments more easily.
Docker evaluates the instructions in a Dockerfile in order. **The
first instruction must be `FROM`** in order to specify the
:ref:`base_image_def` from which you are building.
Docker will treat lines that *begin* with ``#`` as a comment. A ``#``
marker anywhere else in the line will be treated as an argument. This
allows statements like:
::
# Comment
RUN echo 'we are running some # of cool things'
.. _dockerfile_instructions:
Here is the set of instructions you can use in a ``Dockerfile`` for
building images.
.. _dockerfile_from:
``FROM``
========
``FROM <image>``
Or
``FROM <image>:<tag>``
The ``FROM`` instruction sets the :ref:`base_image_def` for subsequent
instructions. As such, a valid Dockerfile must have ``FROM`` as its
first instruction. The image can be any valid image -- it is
especially easy to start by **pulling an image** from the
:ref:`using_public_repositories`.
``FROM`` must be the first non-comment instruction in the
``Dockerfile``.
``FROM`` can appear multiple times within a single Dockerfile in order
to create multiple images. Simply make a note of the last image id
output by the commit before each new ``FROM`` command.
If no ``tag`` is given to the ``FROM`` instruction, ``latest`` is
assumed. If the used tag does not exist, an error will be returned.
.. _dockerfile_maintainer:
``MAINTAINER``
==============
``MAINTAINER <name>``
The ``MAINTAINER`` instruction allows you to set the *Author* field of
the generated images.
.. _dockerfile_run:
``RUN``
=======
RUN has 2 forms:
* ``RUN <command>`` (the command is run in a shell - ``/bin/sh -c``)
* ``RUN ["executable", "param1", "param2"]`` (*exec* form)
The ``RUN`` instruction will execute any commands in a new layer on top
of the current image and commit the results. The resulting committed image
will be used for the next step in the Dockerfile.
Layering ``RUN`` instructions and generating commits conforms to the
core concepts of Docker where commits are cheap and containers can be
created from any point in an image's history, much like source
control.
The *exec* form makes it possible to avoid shell string munging, and to ``RUN``
commands using a base image that does not contain ``/bin/sh``.
Known Issues (RUN)
..................
* :issue:`783` is about file permissions problems that can occur when
using the AUFS file system. You might notice it during an attempt to
``rm`` a file, for example. The issue describes a workaround.
* :issue:`2424` Locale will not be set automatically.
.. _dockerfile_cmd:
``CMD``
=======
CMD has three forms:
* ``CMD ["executable","param1","param2"]`` (like an *exec*, preferred form)
* ``CMD ["param1","param2"]`` (as *default parameters to ENTRYPOINT*)
* ``CMD command param1 param2`` (as a *shell*)
There can only be one CMD in a Dockerfile. If you list more than one
CMD then only the last CMD will take effect.
**The main purpose of a CMD is to provide defaults for an executing
container.** These defaults can include an executable, or they can
omit the executable, in which case you must specify an ENTRYPOINT as
well.
When used in the shell or exec formats, the ``CMD`` instruction sets
the command to be executed when running the image.
If you use the *shell* form of the CMD, then the ``<command>`` will
execute in ``/bin/sh -c``:
.. code-block:: bash
FROM ubuntu
CMD echo "This is a test." | wc -
If you want to **run your** ``<command>`` **without a shell** then you
must express the command as a JSON array and give the full path to the
executable. **This array form is the preferred format of CMD.** Any
additional parameters must be individually expressed as strings in the
array:
.. code-block:: bash
FROM ubuntu
CMD ["/usr/bin/wc","--help"]
If you would like your container to run the same executable every
time, then you should consider using ``ENTRYPOINT`` in combination
with ``CMD``. See :ref:`dockerfile_entrypoint`.
If the user specifies arguments to ``docker run`` then they will
override the default specified in CMD.
.. note::
Don't confuse ``RUN`` with ``CMD``. ``RUN`` actually runs a
command and commits the result; ``CMD`` does not execute anything at
build time, but specifies the intended command for the image.
.. _dockerfile_expose:
``EXPOSE``
==========
``EXPOSE <port> [<port>...]``
The ``EXPOSE`` instructions informs Docker that the container will listen
on the specified network ports at runtime. Docker uses this information
to interconnect containers using links (see :ref:`links <working_with_links_names>`),
and to setup port redirection on the host system (see :ref:`port_redirection`).
.. _dockerfile_env:
``ENV``
=======
``ENV <key> <value>``
The ``ENV`` instruction sets the environment variable ``<key>`` to the
value ``<value>``. This value will be passed to all future ``RUN``
instructions. This is functionally equivalent to prefixing the command
with ``<key>=<value>``
The environment variables set using ``ENV`` will persist when a container is run
from the resulting image. You can view the values using ``docker inspect``, and change them using ``docker run --env <key>=<value>``.
.. note::
One example where this can cause unexpected consequenses, is setting
``ENV DEBIAN_FRONTEND noninteractive``.
Which will persist when the container is run interactively; for example:
``docker run -t -i image bash``
.. _dockerfile_add:
``ADD``
=======
``ADD <src> <dest>``
The ``ADD`` instruction will copy new files from <src> and add them to
the container's filesystem at path ``<dest>``.
``<src>`` must be the path to a file or directory relative to the
source directory being built (also called the *context* of the build) or
a remote file URL.
``<dest>`` is the absolute path to which the source will be copied inside the
destination container.
All new files and directories are created with mode 0755, uid and gid
0.
.. note::
if you build using STDIN (``docker build - < somefile``), there is no build
context, so the Dockerfile can only contain an URL based ADD statement.
.. note::
if your URL files are protected using authentication, you will need to use
an ``RUN wget`` , ``RUN curl`` or other tool from within the container as
ADD does not support authentication.
The copy obeys the following rules:
* The ``<src>`` path must be inside the *context* of the build; you cannot
``ADD ../something /something``, because the first step of a
``docker build`` is to send the context directory (and subdirectories) to
the docker daemon.
* If ``<src>`` is a URL and ``<dest>`` does not end with a trailing slash,
then a file is downloaded from the URL and copied to ``<dest>``.
* If ``<src>`` is a URL and ``<dest>`` does end with a trailing slash,
then the filename is inferred from the URL and the file is downloaded to
``<dest>/<filename>``. For instance, ``ADD http://example.com/foobar /``
would create the file ``/foobar``. The URL must have a nontrivial path
so that an appropriate filename can be discovered in this case
(``http://example.com`` will not work).
* If ``<src>`` is a directory, the entire directory is copied,
including filesystem metadata.
* If ``<src>`` is a *local* tar archive in a recognized compression
format (identity, gzip, bzip2 or xz) then it is unpacked as a
directory. Resources from *remote* URLs are **not** decompressed.
When a directory is copied or unpacked, it has the same behavior as
``tar -x``: the result is the union of
1. whatever existed at the destination path and
2. the contents of the source tree,
with conflicts resolved in favor of "2." on a file-by-file basis.
* If ``<src>`` is any other kind of file, it is copied individually
along with its metadata. In this case, if ``<dest>`` ends with a
trailing slash ``/``, it will be considered a directory and the
contents of ``<src>`` will be written at ``<dest>/base(<src>)``.
* If ``<dest>`` does not end with a trailing slash, it will be
considered a regular file and the contents of ``<src>`` will be
written at ``<dest>``.
* If ``<dest>`` doesn't exist, it is created along with all missing
directories in its path.
.. _dockerfile_entrypoint:
``ENTRYPOINT``
==============
ENTRYPOINT has two forms:
* ``ENTRYPOINT ["executable", "param1", "param2"]`` (like an *exec*,
preferred form)
* ``ENTRYPOINT command param1 param2`` (as a *shell*)
There can only be one ``ENTRYPOINT`` in a Dockerfile. If you have more
than one ``ENTRYPOINT``, then only the last one in the Dockerfile will
have an effect.
An ``ENTRYPOINT`` helps you to configure a container that you can run
as an executable. That is, when you specify an ``ENTRYPOINT``, then
the whole container runs as if it was just that executable.
The ``ENTRYPOINT`` instruction adds an entry command that will **not**
be overwritten when arguments are passed to ``docker run``, unlike the
behavior of ``CMD``. This allows arguments to be passed to the
entrypoint. i.e. ``docker run <image> -d`` will pass the "-d"
argument to the ENTRYPOINT.
You can specify parameters either in the ENTRYPOINT JSON array (as in
"like an exec" above), or by using a CMD statement. Parameters in the
ENTRYPOINT will not be overridden by the ``docker run`` arguments, but
parameters specified via CMD will be overridden by ``docker run``
arguments.
Like a ``CMD``, you can specify a plain string for the ENTRYPOINT and
it will execute in ``/bin/sh -c``:
.. code-block:: bash
FROM ubuntu
ENTRYPOINT wc -l -
For example, that Dockerfile's image will *always* take stdin as input
("-") and print the number of lines ("-l"). If you wanted to make
this optional but default, you could use a CMD:
.. code-block:: bash
FROM ubuntu
CMD ["-l", "-"]
ENTRYPOINT ["/usr/bin/wc"]
.. _dockerfile_volume:
``VOLUME``
==========
``VOLUME ["/data"]``
The ``VOLUME`` instruction will create a mount point with the specified name and mark it
as holding externally mounted volumes from native host or other containers. For more information/examples
and mounting instructions via docker client, refer to :ref:`volume_def` documentation.
.. _dockerfile_user:
``USER``
========
``USER daemon``
The ``USER`` instruction sets the username or UID to use when running
the image.
.. _dockerfile_workdir:
``WORKDIR``
===========
``WORKDIR /path/to/workdir``
The ``WORKDIR`` instruction sets the working directory for the ``RUN``, ``CMD`` and
``ENTRYPOINT`` Dockerfile commands that follow it.
It can be used multiple times in the one Dockerfile. If a relative path is
provided, it will be relative to the path of the previous ``WORKDIR``
instruction. For example:
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
The output of the final ``pwd`` command in this Dockerfile would be ``/a/b/c``.
``ONBUILD``
===========
``ONBUILD [INSTRUCTION]``
The ``ONBUILD`` instruction adds to the image a "trigger" instruction to be
executed at a later time, when the image is used as the base for another build.
The trigger will be executed in the context of the downstream build, as if it
had been inserted immediately after the *FROM* instruction in the downstream
Dockerfile.
Any build instruction can be registered as a trigger.
This is useful if you are building an image which will be used as a base to build
other images, for example an application build environment or a daemon which may be
customized with user-specific configuration.
For example, if your image is a reusable python application builder, it will require
application source code to be added in a particular directory, and it might require
a build script to be called *after* that. You can't just call *ADD* and *RUN* now,
because you don't yet have access to the application source code, and it will be
different for each application build. You could simply provide application developers
with a boilerplate Dockerfile to copy-paste into their application, but that is
inefficient, error-prone and difficult to update because it mixes with
application-specific code.
The solution is to use *ONBUILD* to register in advance instructions to run later,
during the next build stage.
Here's how it works:
1. When it encounters an *ONBUILD* instruction, the builder adds a trigger to
the metadata of the image being built.
The instruction does not otherwise affect the current build.
2. At the end of the build, a list of all triggers is stored in the image manifest,
under the key *OnBuild*. They can be inspected with *docker inspect*.
3. Later the image may be used as a base for a new build, using the *FROM* instruction.
As part of processing the *FROM* instruction, the downstream builder looks for *ONBUILD*
triggers, and executes them in the same order they were registered. If any of the
triggers fail, the *FROM* instruction is aborted which in turn causes the build
to fail. If all triggers succeed, the FROM instruction completes and the build
continues as usual.
4. Triggers are cleared from the final image after being executed. In other words
they are not inherited by "grand-children" builds.
For example you might add something like this:
.. code-block:: bash
[...]
ONBUILD ADD . /app/src
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
[...]
.. warning:: Chaining ONBUILD instructions using `ONBUILD ONBUILD` isn't allowed.
.. warning:: ONBUILD may not trigger FROM or MAINTAINER instructions.
.. _dockerfile_examples:
Dockerfile Examples
======================
.. code-block:: bash
# Nginx
#
# VERSION 0.0.1
FROM ubuntu
MAINTAINER Guillaume J. Charmes <guillaume@docker.com>
# make sure the package repository is up to date
RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update
RUN apt-get install -y inotify-tools nginx apache2 openssh-server
.. code-block:: bash
# Firefox over VNC
#
# VERSION 0.3
FROM ubuntu
# make sure the package repository is up to date
RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update
# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get install -y x11vnc xvfb firefox
RUN mkdir /.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc'
EXPOSE 5900
CMD ["x11vnc", "-forever", "-usepw", "-create"]
.. code-block:: bash
# Multiple images example
#
# VERSION 0.1
FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f
FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4
# You'll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.

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:title: Commands
:description: docker command line interface
:keywords: commands, command line, help, docker
Commands
========
Contents:
.. toctree::
:maxdepth: 1
cli

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:title: Docker Reference Manual
:description: References
:keywords: docker, references, api, command line, commands
.. _references:
Reference Manual
================
Contents:
.. toctree::
:maxdepth: 1
commandline/index
builder
run
api/index

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:title: Docker Run Reference
:description: Configure containers at runtime
:keywords: docker, run, configure, runtime
.. _run_docker:
====================
Docker Run Reference
====================
**Docker runs processes in isolated containers**. When an operator
executes ``docker run``, she starts a process with its own file
system, its own networking, and its own isolated process tree. The
:ref:`image_def` which starts the process may define defaults related
to the binary to run, the networking to expose, and more, but ``docker
run`` gives final control to the operator who starts the container
from the image. That's the main reason :ref:`cli_run` has more options
than any other ``docker`` command.
Every one of the :ref:`example_list` shows running containers, and so
here we try to give more in-depth guidance.
.. _run_running:
General Form
============
As you've seen in the :ref:`example_list`, the basic `run` command
takes this form::
docker run [OPTIONS] IMAGE[:TAG] [COMMAND] [ARG...]
To learn how to interpret the types of ``[OPTIONS]``, see
:ref:`cli_options`.
The list of ``[OPTIONS]`` breaks down into two groups:
1. Settings exclusive to operators, including:
* Detached or Foreground running,
* Container Identification,
* Network settings, and
* Runtime Constraints on CPU and Memory
* Privileges and LXC Configuration
2. Setting shared between operators and developers, where operators
can override defaults developers set in images at build time.
Together, the ``docker run [OPTIONS]`` give complete control over
runtime behavior to the operator, allowing them to override all
defaults set by the developer during ``docker build`` and nearly all
the defaults set by the Docker runtime itself.
Operator Exclusive Options
==========================
Only the operator (the person executing ``docker run``) can set the
following options.
.. contents::
:local:
Detached vs Foreground
----------------------
When starting a Docker container, you must first decide if you want to
run the container in the background in a "detached" mode or in the
default foreground mode::
-d=false: Detached mode: Run container in the background, print new container id
Detached (-d)
.............
In detached mode (``-d=true`` or just ``-d``), all I/O should be done
through network connections or shared volumes because the container is
no longer listening to the commandline where you executed ``docker
run``. You can reattach to a detached container with ``docker``
:ref:`cli_attach`. If you choose to run a container in the detached
mode, then you cannot use the ``--rm`` option.
Foreground
..........
In foreground mode (the default when ``-d`` is not specified),
``docker run`` can start the process in the container and attach the
console to the process's standard input, output, and standard
error. It can even pretend to be a TTY (this is what most commandline
executables expect) and pass along signals. All of that is
configurable::
-a=[] : Attach to ``stdin``, ``stdout`` and/or ``stderr``
-t=false : Allocate a pseudo-tty
--sig-proxy=true: Proxify all received signal to the process (even in non-tty mode)
-i=false : Keep STDIN open even if not attached
If you do not specify ``-a`` then Docker will `attach everything
(stdin,stdout,stderr)
<https://github.com/dotcloud/docker/blob/75a7f4d90cde0295bcfb7213004abce8d4779b75/commands.go#L1797>`_. You
can specify to which of the three standard streams (``stdin``, ``stdout``,
``stderr``) you'd like to connect instead, as in::
docker run -a stdin -a stdout -i -t ubuntu /bin/bash
For interactive processes (like a shell) you will typically want a tty
as well as persistent standard input (``stdin``), so you'll use ``-i
-t`` together in most interactive cases.
Container Identification
------------------------
Name (--name)
.............
The operator can identify a container in three ways:
* UUID long identifier ("f78375b1c487e03c9438c729345e54db9d20cfa2ac1fc3494b6eb60872e74778")
* UUID short identifier ("f78375b1c487")
* Name ("evil_ptolemy")
The UUID identifiers come from the Docker daemon, and if you do not
assign a name to the container with ``--name`` then the daemon will
also generate a random string name too. The name can become a handy
way to add meaning to a container since you can use this name when
defining :ref:`links <working_with_links_names>` (or any other place
you need to identify a container). This works for both background and
foreground Docker containers.
PID Equivalent
..............
And finally, to help with automation, you can have Docker write the
container ID out to a file of your choosing. This is similar to how
some programs might write out their process ID to a file (you've seen
them as PID files)::
--cidfile="": Write the container ID to the file
Network Settings
----------------
::
-n=true : Enable networking for this container
--dns=[] : Set custom dns servers for the container
By default, all containers have networking enabled and they can make
any outgoing connections. The operator can completely disable
networking with ``docker run -n`` which disables all incoming and outgoing
networking. In cases like this, you would perform I/O through files or
STDIN/STDOUT only.
Your container will use the same DNS servers as the host by default,
but you can override this with ``--dns``.
Clean Up (--rm)
---------------
By default a container's file system persists even after the container
exits. This makes debugging a lot easier (since you can inspect the
final state) and you retain all your data by default. But if you are
running short-term **foreground** processes, these container file
systems can really pile up. If instead you'd like Docker to
**automatically clean up the container and remove the file system when
the container exits**, you can add the ``--rm`` flag::
--rm=false: Automatically remove the container when it exits (incompatible with -d)
Runtime Constraints on CPU and Memory
-------------------------------------
The operator can also adjust the performance parameters of the container::
-m="": Memory limit (format: <number><optional unit>, where unit = b, k, m or g)
-c=0 : CPU shares (relative weight)
The operator can constrain the memory available to a container easily
with ``docker run -m``. If the host supports swap memory, then the
``-m`` memory setting can be larger than physical RAM.
Similarly the operator can increase the priority of this container
with the ``-c`` option. By default, all containers run at the same
priority and get the same proportion of CPU cycles, but you can tell
the kernel to give more shares of CPU time to one or more containers
when you start them via Docker.
Runtime Privilege and LXC Configuration
---------------------------------------
::
--privileged=false: Give extended privileges to this container
--lxc-conf=[]: (lxc exec-driver only) Add custom lxc options --lxc-conf="lxc.cgroup.cpuset.cpus = 0,1"
By default, Docker containers are "unprivileged" and cannot, for
example, run a Docker daemon inside a Docker container. This is
because by default a container is not allowed to access any devices,
but a "privileged" container is given access to all devices (see
lxc-template.go_ and documentation on `cgroups devices
<https://www.kernel.org/doc/Documentation/cgroups/devices.txt>`_).
When the operator executes ``docker run --privileged``, Docker will
enable to access to all devices on the host as well as set some
configuration in AppArmor to allow the container nearly all the same
access to the host as processes running outside containers on the
host. Additional information about running with ``--privileged`` is
available on the `Docker Blog
<http://blog.docker.io/2013/09/docker-can-now-run-within-docker/>`_.
If the Docker daemon was started using the ``lxc`` exec-driver
(``docker -d --exec-driver=lxc``) then the operator can also specify
LXC options using one or more ``--lxc-conf`` parameters. These can be
new parameters or override existing parameters from the lxc-template.go_.
Note that in the future, a given host's Docker daemon may not use LXC,
so this is an implementation-specific configuration meant for operators
already familiar with using LXC directly.
.. _lxc-template.go: https://github.com/dotcloud/docker/blob/master/execdriver/lxc/lxc_template.go
Overriding ``Dockerfile`` Image Defaults
========================================
When a developer builds an image from a :ref:`Dockerfile
<dockerbuilder>` or when she commits it, the developer can set a
number of default parameters that take effect when the image starts up
as a container.
Four of the ``Dockerfile`` commands cannot be overridden at runtime:
``FROM, MAINTAINER, RUN``, and ``ADD``. Everything else has a
corresponding override in ``docker run``. We'll go through what the
developer might have set in each ``Dockerfile`` instruction and how the
operator can override that setting.
.. contents::
:local:
CMD (Default Command or Options)
--------------------------------
Recall the optional ``COMMAND`` in the Docker commandline::
docker run [OPTIONS] IMAGE[:TAG] [COMMAND] [ARG...]
This command is optional because the person who created the ``IMAGE``
may have already provided a default ``COMMAND`` using the ``Dockerfile``
``CMD``. As the operator (the person running a container from the
image), you can override that ``CMD`` just by specifying a new
``COMMAND``.
If the image also specifies an ``ENTRYPOINT`` then the ``CMD`` or
``COMMAND`` get appended as arguments to the ``ENTRYPOINT``.
ENTRYPOINT (Default Command to Execute at Runtime
-------------------------------------------------
::
--entrypoint="": Overwrite the default entrypoint set by the image
The ENTRYPOINT of an image is similar to a ``COMMAND`` because it
specifies what executable to run when the container starts, but it is
(purposely) more difficult to override. The ``ENTRYPOINT`` gives a
container its default nature or behavior, so that when you set an
``ENTRYPOINT`` you can run the container *as if it were that binary*,
complete with default options, and you can pass in more options via
the ``COMMAND``. But, sometimes an operator may want to run something else
inside the container, so you can override the default ``ENTRYPOINT`` at
runtime by using a string to specify the new ``ENTRYPOINT``. Here is an
example of how to run a shell in a container that has been set up to
automatically run something else (like ``/usr/bin/redis-server``)::
docker run -i -t --entrypoint /bin/bash example/redis
or two examples of how to pass more parameters to that ENTRYPOINT::
docker run -i -t --entrypoint /bin/bash example/redis -c ls -l
docker run -i -t --entrypoint /usr/bin/redis-cli example/redis --help
EXPOSE (Incoming Ports)
-----------------------
The ``Dockerfile`` doesn't give much control over networking, only
providing the ``EXPOSE`` instruction to give a hint to the operator
about what incoming ports might provide services. The following
options work with or override the ``Dockerfile``'s exposed defaults::
--expose=[]: Expose a port from the container
without publishing it to your host
-P=false : Publish all exposed ports to the host interfaces
-p=[] : Publish a container's port to the host (format:
ip:hostPort:containerPort | ip::containerPort |
hostPort:containerPort)
(use 'docker port' to see the actual mapping)
--link="" : Add link to another container (name:alias)
As mentioned previously, ``EXPOSE`` (and ``--expose``) make a port
available **in** a container for incoming connections. The port number
on the inside of the container (where the service listens) does not
need to be the same number as the port exposed on the outside of the
container (where clients connect), so inside the container you might
have an HTTP service listening on port 80 (and so you ``EXPOSE 80`` in
the ``Dockerfile``), but outside the container the port might be 42800.
To help a new client container reach the server container's internal
port operator ``--expose``'d by the operator or ``EXPOSE``'d by the
developer, the operator has three choices: start the server container
with ``-P`` or ``-p,`` or start the client container with ``--link``.
If the operator uses ``-P`` or ``-p`` then Docker will make the
exposed port accessible on the host and the ports will be available to
any client that can reach the host. To find the map between the host
ports and the exposed ports, use ``docker port``)
If the operator uses ``--link`` when starting the new client container,
then the client container can access the exposed port via a private
networking interface. Docker will set some environment variables in
the client container to help indicate which interface and port to use.
ENV (Environment Variables)
---------------------------
The operator can **set any environment variable** in the container by
using one or more ``-e`` flags, even overriding those already defined by the
developer with a Dockefile ``ENV``::
$ docker run -e "deep=purple" --rm ubuntu /bin/bash -c export
declare -x HOME="/"
declare -x HOSTNAME="85bc26a0e200"
declare -x OLDPWD
declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
declare -x PWD="/"
declare -x SHLVL="1"
declare -x container="lxc"
declare -x deep="purple"
Similarly the operator can set the **hostname** with ``-h``.
``--link name:alias`` also sets environment variables, using the
*alias* string to define environment variables within the container
that give the IP and PORT information for connecting to the service
container. Let's imagine we have a container running Redis::
# Start the service container, named redis-name
$ docker run -d --name redis-name dockerfiles/redis
4241164edf6f5aca5b0e9e4c9eccd899b0b8080c64c0cd26efe02166c73208f3
# The redis-name container exposed port 6379
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
4241164edf6f dockerfiles/redis:latest /redis-stable/src/re 5 seconds ago Up 4 seconds 6379/tcp redis-name
# Note that there are no public ports exposed since we didn't use -p or -P
$ docker port 4241164edf6f 6379
2014/01/25 00:55:38 Error: No public port '6379' published for 4241164edf6f
Yet we can get information about the Redis container's exposed ports
with ``--link``. Choose an alias that will form a valid environment
variable!
::
$ docker run --rm --link redis-name:redis_alias --entrypoint /bin/bash dockerfiles/redis -c export
declare -x HOME="/"
declare -x HOSTNAME="acda7f7b1cdc"
declare -x OLDPWD
declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
declare -x PWD="/"
declare -x REDIS_ALIAS_NAME="/distracted_wright/redis"
declare -x REDIS_ALIAS_PORT="tcp://172.17.0.32:6379"
declare -x REDIS_ALIAS_PORT_6379_TCP="tcp://172.17.0.32:6379"
declare -x REDIS_ALIAS_PORT_6379_TCP_ADDR="172.17.0.32"
declare -x REDIS_ALIAS_PORT_6379_TCP_PORT="6379"
declare -x REDIS_ALIAS_PORT_6379_TCP_PROTO="tcp"
declare -x SHLVL="1"
declare -x container="lxc"
And we can use that information to connect from another container as a client::
$ docker run -i -t --rm --link redis-name:redis_alias --entrypoint /bin/bash dockerfiles/redis -c '/redis-stable/src/redis-cli -h $REDIS_ALIAS_PORT_6379_TCP_ADDR -p $REDIS_ALIAS_PORT_6379_TCP_PORT'
172.17.0.32:6379>
VOLUME (Shared Filesystems)
---------------------------
::
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "container-dir" is missing, then docker creates a new volume.
--volumes-from="": Mount all volumes from the given container(s)
The volumes commands are complex enough to have their own
documentation in section :ref:`volume_def`. A developer can define one
or more ``VOLUME``\s associated with an image, but only the operator can
give access from one container to another (or from a container to a
volume mounted on the host).
USER
----
The default user within a container is ``root`` (id = 0), but if the
developer created additional users, those are accessible too. The
developer can set a default user to run the first process with the
``Dockerfile USER`` command, but the operator can override it ::
-u="": Username or UID
WORKDIR
-------
The default working directory for running binaries within a container is the root directory (``/``), but the developer can set a different default with the ``Dockerfile WORKDIR`` command. The operator can override this with::
-w="": Working directory inside the container