Fix docker uid

[TarEnethil]

Allow the user to override the uid of the 'dungeonmaster' user inside the docker container using the environment variable USER_ID=...

This results in several changes to the structure of the Dockerfile and entrypoint.sh:

• remove USER directive in dockerfile, meaning we enter entrypoint.sh as root
• (if necessary) change the uid of dungeonmaster and re-chown the homedir
• all commands inside the entrypoint are now executed with su-exec, which runs them as dungeonmaster.
• check that the dungeonmaster-user is the owner of the /opt/data mount, error otherwise.

Closes #99.