For nehe.sr.

I think that https://gitlab.com/gitlab-org/gitlab-runner/-/issues/1736
is solved and the version I have deployed has the fix.

Use the Docker snippet.

- Use an environment variable for setting the location of the dhparams
file.
- Don't generate the file if it already exists.
- Use the dhparams file in the host in production.

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.