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General cleanup of the builder.md file 

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
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James Turnbull 2014-07-11 22:28:36 -04:00 committed by Tibor Vass
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docs/sources/reference/builder.md
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@ -4,15 +4,17 @@ page_keywords: builder, docker, Dockerfile, automation, image creation
# 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.
**Docker can build images automatically** by reading the instructions
from a `Dockerfile`. A `Dockerfile` is a text document that contains all
the commands you would normally execute manually in order to build a
Docker image. By calling `docker build` from your terminal, you can have
Docker build your image step by step, executing the instructions
successively.
## Usage
To [*build*](../commandline/cli/#cli-build) an image from a source repository,
create a description file called Dockerfile at the root of your 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 the argument
@ -55,13 +57,12 @@ accelerating `docker build` significantly (indicated by `Using cache`):
---> 1a5ffc17324d
Successfully built 1a5ffc17324d
When you're done with your build, you're ready to look into
[*Pushing a repository to its registry*](
/userguide/dockerrepos/#image-push).
When you're done with your build, you're ready to look into [*Pushing a
repository to its registry*]( /userguide/dockerrepos/#image-push).
## Format
Here is the format of the Dockerfile:
Here is the format of the `Dockerfile`:
# Comment
INSTRUCTION arguments
@ -69,8 +70,8 @@ Here is the format of the Dockerfile:
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 [*Base
Docker runs the instructions in a `Dockerfile` in order. **The
first instruction must be \`FROM\`** in order to specify the [*Base
Image*](/terms/image/#base-image-def) from which you are building.
Docker will treat lines that *begin* with `#` as a
@ -80,10 +81,10 @@ be treated as an argument. This allows statements like:
# Comment
RUN echo 'we are running some # of cool things'
Here is the set of instructions you can use in a Dockerfile
for building images.
Here is the set of instructions you can use in a `Dockerfile` for building
images.
## .dockerignore
## The `.dockerignore` file
If a file named `.dockerignore` exists in the source repository, then it
is interpreted as a newline-separated list of exclusion patterns.
@ -124,15 +125,15 @@ Or
FROM <image>:<tag>
The `FROM` instruction sets the [*Base Image*](/terms/image/#base-image-def)
for subsequent instructions. As such, a valid Dockerfile must have `FROM` as
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 [*Public Repositories*](
/userguide/dockerrepos/#using-public-repositories).
`FROM` must be the first non-comment instruction in the Dockerfile.
`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
`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
@ -154,7 +155,7 @@ RUN has 2 forms:
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.
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
@ -163,11 +164,11 @@ 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`.
The cache for `RUN` instructions isn't invalidated automatically during the
next build. The cache for an instruction like `RUN apt-get dist-upgrade -y`
will be reused during the next build.
The cache for `RUN` instructions can be invalidated by using the `--no-cache`
flag, for example `docker build --no-cache`.
The cache for `RUN` instructions isn't invalidated automatically during
the next build. The cache for an instruction like `RUN apt-get
dist-upgrade -y` will be reused during the next build. The cache for
`RUN` instructions can be invalidated by using the `--no-cache` flag,
for example `docker build --no-cache`.
The cache for `RUN` instructions can be invalidated by `ADD` instructions. See
[below](#add) for details.
@ -178,28 +179,27 @@ The cache for `RUN` instructions can be invalidated by `ADD` instructions. See
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](https://github.com/dotcloud/docker/issues/2424) Locale will
not be set automatically.
## CMD
CMD has three forms:
The `CMD` instruction has three forms:
- `CMD ["executable","param1","param2"]` (like an *exec*, this is the 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.
There can only be one `CMD` instruction 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
**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.
the executable, in which case you must specify an `ENTRYPOINT`
instruction 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
If you use the *shell* form of the `CMD`, then the `<command>` will execute in
`/bin/sh -c`:
FROM ubuntu
@ -207,7 +207,7 @@ If you use the *shell* form of the CMD, then the `<command>` will execute in
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
**This array form is the preferred format of `CMD`.** Any additional parameters
must be individually expressed as strings in the array:
FROM ubuntu
@ -218,7 +218,7 @@ you should consider using `ENTRYPOINT` in combination with `CMD`. See
[*ENTRYPOINT*](#entrypoint).
If the user specifies arguments to `docker run` then they will override the
default specified in CMD.
default specified in `CMD`.
> **Note**:
> don't confuse `RUN` with `CMD`. `RUN` actually runs a command and commits
@ -264,24 +264,23 @@ 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 a uid and gid of 0.
All new files and directories are created with a UID and GID of 0.
In the case where `<src>` is a remote file URL, the destination will have permissions 600.
In the case where `<src>` is a remote file URL, the destination will
have permissions of 600.
> **Note**:
> If you build by passing a Dockerfile through STDIN (`docker build - < somefile`),
> there is no build context, so the Dockerfile can only contain a URL
> based ADD statement.
> If you build by passing a `Dockerfile` through STDIN (`docker
> build - < somefile`), there is no build context, so the `Dockerfile`
> can only contain a URL based `ADD` instruction. You can also pass a
> compressed archive through STDIN: (`docker build - < archive.tar.gz`),
> the `Dockerfile` at the root of the archive and the rest of the
> archive will get used at the context of the build.
> You can also pass a compressed archive through STDIN:
> (`docker build - < archive.tar.gz`), the `Dockerfile` at the root of
> the archive and the rest of the archive will get used at the context
> of the build.
>
> **Note**:
> If your URL files are protected using authentication, you will need to
> use `RUN wget` , `RUN curl`
> or use another tool from within the container as ADD does not support
> If your URL files are protected using authentication, you
> will need to use `RUN wget`, `RUN curl` or use another tool from
> within the container as the `ADD` instruction does not support
> authentication.
> **Note**:
@ -314,9 +313,9 @@ The copy obeys the following rules:
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.
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
@ -342,7 +341,7 @@ being built (also called the *context* of the build).
`<dest>` is the absolute path to which the source will be copied inside the
destination container.
All new files and directories are created with a uid and gid of 0.
All new files and directories are created with a UID and GID of 0.
> **Note**:
> If you build using STDIN (`docker build - < somefile`), there is no
@ -431,34 +430,34 @@ instructions via the Docker client, refer to [*Share Directories via Volumes*](
USER daemon
The `USER` instruction sets the username or UID to use when running the image
The `USER` instruction sets the user name or UID to use when running the image
and for any following `RUN` directives.
## WORKDIR
WORKDIR /path/to/workdir
The `WORKDIR` instruction sets the working directory for the `RUN`, `CMD` and
`ENTRYPOINT` Dockerfile commands that follow it.
The `WORKDIR` instruction sets the working directory for any `RUN`, `CMD` and
`ENTRYPOINT` instructions that follow it in the `Dockerfile`.
It can be used multiple times in the one Dockerfile. If a relative path
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`.
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.
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.
@ -466,33 +465,33 @@ 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
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
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
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
The solution is to use `ONBUILD` to register 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
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*.
image manifest, under the key `OnBuild`. They can be inspected with
the `docker inspect` command.
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
`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
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.
@ -504,9 +503,9 @@ For example you might add something like this:
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
[...]
> **Warning**: Chaining ONBUILD instructions using ONBUILD ONBUILD isn't allowed.
> **Warning**: Chaining `ONBUILD` instructions using `ONBUILD ONBUILD` isn't allowed.
> **Warning**: ONBUILD may not trigger FROM or MAINTAINER instructions.
> **Warning**: The `ONBUILD` instruction may not trigger `FROM` or `MAINTAINER` instructions.
## Dockerfile Examples
@ -557,3 +556,4 @@ For example you might add something like this:
# You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.