docker: Allowing to downgrade to regular user via environment variables

[tuxuser]

This change implements an entrypoint-script for the docker container.

By passing environment variables PUID=<numeric user id> and PGID=<numeric group id> to the container, which is started as root by the docker daemon, can downgrade to the respective user and run the stump server daemon with lower privileges.

This also ensures that mounted directories/volumes are assigned and accessed with proper permissions.

NOTE: This essentially breaks the invocation via --user argument, currently documented on Docker hub

• Fixes issue Docker permissions issue #56

Example docker compose

version: '3.3'
services:
  stump:
    image: aaronleopold/stump-preview:latest
    container_name: stump
    volumes:
      - /home/user/.stump-config:/config
      - /media/books:/data
    ports:
      - 10801:10801
    environment:
      - PUID=1000
      - PGID=1000
    restart: unless-stopped

or via CLI

docker create \
  --name=stump \
  -e "PUID=1000" \
  -e "PGID=1000" \
  -p 6969:6969 \
  --mount type=bind,source=/Users/aaronleopold/.stump,target=/config \
  --mount type=bind,source=/Users/aaronleopold/Documents/Stump,target=/data \
  --restart unless-stopped \
  aaronleopold/stump-preview

[aaronleopold]

Thanks so much, @tuxuser! I only had one comment, it looks great otherwise! I'm building an image now and can report back one I test it out

[aaronleopold]

All looks good! I'll merge this tomorrow 🚀