Store a ref to a custom Dockerfile, rather than the file itself

[amarburg]

It would be nice if the custom Dockerfile could live in the src/ repo (in a git repo), rather than an untracked copy in the ROS workspace. This could be done with a symlink. Another option could change the syntax of the docker_override file to support either a Docker tag or a path to a Dockerfile.

[amarburg]

Come to think of it, this could be done with a symlink (?)