Supervision/management should not be done from inside the container, but
externally, by container managers.
The container now simply runs node on server.js.
The logs are now readable from docker logs <container_name>.
This is a super simple start.
At minimum, configuration via environment variables (see #3543) needs to be
integrated in Etherpad to make this user-friendly.
Resolves#3524.
Samuel Lelièvre