…od#8.

* Debian is the recommended base image for Docker.
  See https://docs.docker.com/articles/dockerfile_best-practices/#from
* Changed volume of tor home dir to the default one /var/lib/tor.
  Note that when I do `docker rm … && docker run …` docker will not use
  the previously used volume but instead create a new container
  resulting in a new private key being generated.
  I use `docker run -v /srv/tor:/var/lib/tor` for persistent storage.
* https://www.torproject.org/docs/debian.html.en
* apt automatically checks packages with GPG. Related to patrickod#8.
* Using apt_preferences to ensure that packages from deb.torproject.org are
  preferred. Without this, all packages are installed from the Debian repository.
  See `man apt_preferences`.
* One could also run tor inside the Docker container as debian-tor user.
  But note that the UID of debian-tor might be mapped to a different user
  outside of the container resulting in read+write access for this user
  to the private key.