Preperations for registry backup.

I only have off-site backups for host01. In case I need to rebuild ns4
or rebuild shore.co.il infrastructure I need those images (although I
can probably build them locally and push them, it's probably easier and
faster to restore from backups). So this commit includes:

- Empty Python script.
- pre-commit hooks for Python.
- Dockerfile for the image (with the requirements and script).
- GitLab CI job for backup on host01 (bind mount the dockerd socket and
  /var/backups) and a notification job in case the backup fails.

All that's left is to write the backup script.