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.

[ndeloof]

Example request for such feature: https://github.com/docker/compose/issues/4081#issuecomment-1937787722 I 100% agree not being totally confident with such a non-portable feature

[ndeloof]

Let's close this one as lack of portability is challenging here