Try to make GitLab backups more reliable

Sometimes we'd get failures if files changed while backing them up.
See https://docs.gitlab.com/ee/raketasks/backup_restore.html#backup-strategy-option
for documentation on the fix.
This fixes #200.

roles/borg_client/templates/backup-gitlab.sh.j2

@@ -12,4 +12,4 @@ echo "emptying backup directory ${backupdir}"
 # Verify that the gitlab_backupdir in ansible was defined, otherwise we will rm /* and remove the previous backup
 rm -r "${backupdir:?backup dir unset}/"*
 
-docker exec gitlab gitlab-backup create SKIP=tar
+docker exec gitlab gitlab-backup create STRATEGY=copy SKIP=tar