Because for some reason Docker Compose changed from using underscores to
hyphens.

Also, save the last month of backups.

GitLab stores backups as tar files with the date as part of the
filename. Backups older than 10 days are deleted. It's easier for me to
them have off-site backups if they're under /var/backups and there's no
need to have a process to copy or snapshot to that directory (the
current format is fine).