USER support for ARG variable

[tcrawfo3]

The USER instruction in a Dockerfile supports the use of an ARG variable. However, dockerfile_lint complains that this is invalid. I took an example syntax from the Dockerfile reference found at https://docs.docker.com/engine/reference/builder/. Oh, and your reference below is no longer valid.

FROM busybox
USER ${user:-some_user}
ARG user
USER $user

Line 2: -> USER ${user:-some_user} ERROR: Invalid parameters for command.. Reference -> https://docs.docker.com/reference/builder/

[robertauer]

This issue still persists.