expose implicit UID and GID variables

[ndeloof]

A very common issue using compose is to bind mount local files into container, and get into permission issues. Until docker engine provides some uid+gid translation on bind mounts (maybe using bindfs?) the only option is to run container with same user as the owner of the bind-mounted file This introduce support for UID and GID variables on systems this makes sense (i.e. everything but Windows) so one can set user: ${UID}:${GID} in a compose file.