What is the advantage compared to just doing:

volumes:
  - /data/lib:/var/lib/samba
  - /data/cache:/var/cache/samba

Because that would require no changes and seems a bit more clear?

The problem here is, that you must know the structure of /var/lib/samba. The cache folder folder can be mounted, you are right.
I started a blank container:

podman run --name test --rm -ti --entrypoint  /bin/sh ghcr.io/dockur/samba:4.21.4
ls -hlA /var/lib/samba/
total 12K    
drwxrwx---    2 root     root        4.0K Mar 28 01:41 bind-dns
drwx------    2 root     root        4.0K Mar 28 01:41 private
drwxr-xr-x    2 root     root        4.0K Mar 28 01:41 sysvol

If you mount the folder, it will be empty and samba won't start (I tested yesterday). For sure, you can create the folders before starting the container, but not everbody will know how to find out, which folders have to be created.
As long samba does not need more subfolders under /var/lib/samba the once copy at the beginning would be okay...

Maybe it would be better, to find -type d instead of creating folders inside mount if nonexisting.

If you "accept" the issue, I would optimize it a little bit to be future proof.

I expected that Samba would create the folders automaticly. In that case I prefer that the script just creates /var/lib/samba/bind-dns, /var/lib/samba/private and /var/lib/samba/sysvol if they don't exist.

Can you test if it works with the change I made?

I moved the folder creation, because the creation of users can only be done, if the folder structure is given

Apr 20 11:37:08 server samba-app[3697873]: tdbsam_open: Failed to open/create TDB passwd [/var/lib/samba/private/passdb.tdb]
Apr 20 11:37:08 server samba-app[3697873]: tdbsam_getsampwnam: failed to open /var/lib/samba/private/passdb.tdb!
Apr 20 11:37:08 server samba-app[3697873]: User Search failed!
Apr 20 11:37:08 server samba-app[3697873]: Failed to open /var/lib/samba/private/secrets.tdb

Now it will works :)