Basically, just what I did since the last commit. Includes the Exim4
SMTP server.

- Run as a limited user.
- Use a higher number port (for the limited user).
- Compile rules for better performance.
- Add a script to update the rules, recompile and reload the daemon.

- Sort keys in docker-compose.yml.
- Use Docker Hub images for caching and name the images the same were
applicable.
- Mark the volumes for backup.

- Added pre-commit hooks.
- Silence a few warnings.