@@ -273,8 +273,15 @@ A Dockerfile is similar to a Makefile.

 **USER**

   -- `USER daemon`

-  The **USER** instruction sets the username or UID that is used when running the

-  image.

+  Sets the username or UID used for running subsequent commands.

+

+  The **USER** instruction can optionally be used to set the group or GID. The

+  followings examples are all valid:

+  USER [user | user:group | uid | uid:gid | user:gid | uid:group ]

+

+  Until the **USER** instruction is set, instructions will be run as root. The USER

+  instruction can be used any number of times in a Dockerfile, and will only affect

+  subsequent commands.

 **WRKDIR**

   -- `WORKDIR /path/to/workdir`

@@ -341,7 +341,12 @@ The **-t** option is incompatible with a redirection of the docker client

 standard input.

 **-u**, **--user**=""

-   Username or UID

+   Sets the username or UID used and optionally the groupname or GID for the specified command.

+

+   The followings examples are all valid:

+   --user [user | user:group | uid | uid:gid | user:gid | uid:group ]

+

+   Without this argument the command will be run as root in the container.

 **-v**, **--volume**=[]

    Bind mount a volume (e.g., from the host: -v /host:/container, from Docker: -v /container)