Fix the backup pruning Cron job.

- The command silently failed for ages (the container name was missing).
  When I had less information in GitLab it wasn't noticeable, but that's
no longer the case.
- Delete backups that haven't been accessed instead of modified. In case
  I did get something from that backup, keep it around for a while
longer.
- Keep backups for 10 days, in case it's been a busy week or over the
  weekend or something.