Except you never do that (having your DB directory be in the container rootfs). Nobody does, because then (beside the performance/reliability impact you mention

Except you never do that (having your DB directory be in the container rootfs). Nobody does, because then (beside the performance/reliability impact you mention) if the container goes (docker rm) the data goes. You're always gonna use a Volume for this kind of cases, be it K8s volume or Docker volume, and these, as neighbour message mentions, are just bind mounts (or actual mounts in certain cases), so no layers, no overlay, nothing of the sort.