Add docs for Dockerfile ADD/COPY --chown flag

Document the new `--chown` flag added to the ADD and COPY commands in
the Dockerfile format.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

docs/reference/builder.md

@@ -817,16 +817,23 @@ change them using `docker run --env <key>=<value>`.
 
 ADD has two forms:
 
-- `ADD <src>... <dest>`
-- `ADD ["<src>",... "<dest>"]` (this form is required for paths containing
+- `ADD [--chown=<user>:<group>] <src>... <dest>`
+- `ADD [--chown=<user>:<group>] ["<src>",... "<dest>"]` (this form is required for paths containing
 whitespace)
 
+> **Note**:
+> The `--chown` feature is only supported on Dockerfiles used to build Linux containers,
+> and will not work on Windows containers. Since user and group ownership concepts do
+> not translate between Linux and Windows, the use of `/etc/passwd` and `/etc/group` for
+> translating user and group names to IDs restricts this feature to only be viable for
+> for Linux OS-based containers.
+
 The `ADD` instruction copies new files, directories or remote file URLs from `<src>`
 and adds them to the filesystem of the image at the path `<dest>`.
 
-Multiple `<src>` resource may be specified but if they are files or
-directories then they must be relative to the source directory that is
-being built (the context of the build).
+Multiple `<src>` resources may be specified but if they are files or
+directories, their paths are interpreted as relative to the source of
+the context of the build.
 
 Each `<src>` may contain wildcards and matching will be done using Go's
 [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example:
@@ -848,7 +855,26 @@ named `arr[0].txt`, use the following;
     ADD arr[[]0].txt /mydir/    # copy a file named "arr[0].txt" to /mydir/
 
 
-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, unless the
+optional `--chown` flag specifies a given username, groupname, or UID/GID
+combination to request specific ownership of the content added. The
+format of the `--chown` flag allows for either username and groupname strings
+or direct integer UID and GID in any combination. Providing a username without
+groupname or a UID without GID will use the same numeric UID as the GID. If a
+username or groupname is provided, the container's root filesystem
+`/etc/passwd` and `/etc/group` files will be used to perform the translation
+from name to integer UID or GID respectively. The following examples show
+valid definitions for the `--chown` flag:
+
+    ADD --chown=55:mygroup files* /somedir/
+    ADD --chown=bin files* /somedir/
+    ADD --chown=1 files* /somedir/
+    ADD --chown=10:11 files* /somedir/
+
+If the container root filesystem does not contain either `/etc/passwd` or
+`/etc/group` files and either user or group names are used in the `--chown`
+flag, the build will fail on the `ADD` operation. Using numeric IDs requires
+no lookup and will not depend on container root filesystem content.
 
 In the case where `<src>` is a remote file URL, the destination will
 have permissions of 600. If the remote file being retrieved has an HTTP
@@ -938,15 +964,23 @@ guide](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practi
 
 COPY has two forms:
 
-- `COPY <src>... <dest>`
-- `COPY ["<src>",... "<dest>"]` (this form is required for paths containing
+- `COPY [--chown=<user>:<group>] <src>... <dest>`
+- `COPY [--chown=<user>:<group>] ["<src>",... "<dest>"]` (this form is required for paths containing
 whitespace)
 
+> **Note**:
+> The `--chown` feature is only supported on Dockerfiles used to build Linux containers,
+> and will not work on Windows containers. Since user and group ownership concepts do
+> not translate between Linux and Windows, the use of `/etc/passwd` and `/etc/group` for
+> translating user and group names to IDs restricts this feature to only be viable for
+> for Linux OS-based containers.
+
 The `COPY` instruction copies new files or directories from `<src>`
 and adds them to the filesystem of the container at the path `<dest>`.
 
-Multiple `<src>` resource may be specified but they must be relative
-to the source directory that is being built (the context of the build).
+Multiple `<src>` resources may be specified but the paths of files and
+directories will be interpreted as relative to the source of the context
+of the build.
 
 Each `<src>` may contain wildcards and matching will be done using Go's
 [filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example:
@@ -968,7 +1002,26 @@ named `arr[0].txt`, use the following;
 
     COPY arr[[]0].txt /mydir/    # copy a file named "arr[0].txt" to /mydir/
 
-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, unless the
+optional `--chown` flag specifies a given username, groupname, or UID/GID
+combination to request specific ownership of the copied content. The
+format of the `--chown` flag allows for either username and groupname strings
+or direct integer UID and GID in any combination. Providing a username without
+groupname or a UID without GID will use the same numeric UID as the GID. If a
+username or groupname is provided, the container's root filesystem
+`/etc/passwd` and `/etc/group` files will be used to perform the translation
+from name to integer UID or GID respectively. The following examples show
+valid definitions for the `--chown` flag:
+
+    COPY --chown=55:mygroup files* /somedir/
+    COPY --chown=bin files* /somedir/
+    COPY --chown=1 files* /somedir/
+    COPY --chown=10:11 files* /somedir/
+
+If the container root filesystem does not contain either `/etc/passwd` or
+`/etc/group` files and either user or group names are used in the `--chown`
+flag, the build will fail on the `COPY` operation. Using numeric IDs requires
+no lookup and will not depend on container root filesystem content.
 
 > **Note**:
 > If you build using STDIN (`docker build - < somefile`), there is no