Allow custom user and group

[Sytten]

It is a good practice to drop permissions inside the docker image for security reasons. It would be nice if we could set the PUID and the PGID as environment variables.

[scottmando2000]

Bump on this issue

[meonkeys]

This looks a bit more complex than simply forcing a different user and group. The server appears to expect a usable home dir and shell. I'll see about forcing a different user, but it won't be pretty.

[meonkeys]

https://gitlab.com/-/snippets/3679226 proves the concept. Like I said, not pretty. But it works: just specify the user and goup as 50121 on the host when you start the container.