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 similar to using `id` command, we can use `usermod` command to set the users uid and gid

```bash
groupmod -g 500 git  # changes git group gid to 500
usermod -u 500 git    # changes git user uid to 500
```
Using `usermod` and `groupmod` commands will ensure that two groups or users do not have the same id, which could otherwise have unnecessary consequencies.

In the documentation we could add that it is generally safe to ids `500` and above. Newer distros use `1000` and above for system user ids.