Proper user&permission management

[5andr0]

Changelog:

• changed user management based on Best practices for writing Dockerfiles

• it's better to set a consistent default uid/gid to avoid mismatches with future builds when upgrading

• it's necessary to create the group/user beforehand instead by installing packages (there was a different uid/gid set for the /var/run/prosody dir in the latest build)

• instead of gosu for root step-down we can use setpriv on debian

• changed parent image to 'debian:stable-slim' - no need for a bloated image. no need to change the codename anymore when a new stable debian is released

• Maintainer is deprecated using LABEL maintainer instead

The entrypoint.sh will automatically adjust permissions for imported files with another uid/gid.
If you want to run prosody as a specific user you can do this:

docker-compose:

tmpfs:
  - /run/prosody:uid=1000,gid=1000
user: 1000:1000

docker run cli:
docker run --user="1000:1000" --tmpfs /run/prosody:uid=1000,gid=1000 ...

But another user won't have access to /run/prosody
You either have to mount /run/prosody with the users uid/gid like above or change the path of the pidfile to one of your other mounted directories