init: uid/gid adaption

Thanks for this dockerfile :)

I'm running a small gitlab docker-environment together with a few other non dockerized services on a server. The repositories are located in git's home under /home/git/data on the host. This change allows me to make sure that the user git on the host owns all the files in /home/git/data which is mounted into the container.
Otherwise they would be owned by 1000:1000 which belongs to a completely different user. A nice (but unimportant) side effect is that a ps on the host shows that related daemons are running as user git (instead of the user associated with 1000). More details can be found in the commit message.

Since this change might be interesting for other users as well I've created this PR.
@fetzerch Would it make sense to automatically set `USERMAP_GID` to `USERMAP_UID` when `USERMAP_GID` is not specified?