Good question. I have the same setup on one server hosting GitLab, Pi-Hole, Plex, etc., and I have Restic (and its cronjob) installed on the host and only backup the files that I mount to each Docker container, which are all stored in /srv/docker.
In theory, you need to be ready to literally delete every container at any time and pull them from scratch and be 100% fine, since all of your actual data should be stored on the host and mounted as Docker volumes [0]. It's a good Doomsday test if you're looking for one. ;)