Logfile should be exposable to a volume

[Boldie]

Sometimes the command fails with
Error: ./update_mirror.sh buildroot (more information in /opt/gitlab-mirror/cron.log)

but now where to fetch the logs. A mount of the /opt/gitlab-mirror is not good since it contains essential stuff and thus the logfile will vanish on exit. So it would be nice to move the logfile to a different position and mount it with a volume.