I have migrated the NextCloud application from the package that is installed in the QTS itself (downloaded from the club repo) to NextCloud in the docker. It worked and the base is kept at native NAS MariaDB all the time.
On the NAS itself (and not in the docker) there are also files of documents, photos etc. Everything seems to be synchronized with, for example, the NextCloud Linux client. Similarly, access from the Linux filemanager to the files in the database works (via WEBDav).
However, for clients, access to the database is as if in read only mode. It is 100% a problem with some access permissions.

Can my colleagues tell me what to check?

You need to set your UID/GID docker variables to match your filesystem.

So for example, I installed nextcloud from https://hub.docker.com/r/linuxserver/nextcloud/ and had to configure the PUID and PGID variables to match the the permissions on my NAS. You can probably get the uid/gid by running ls -ln in the file share of your NAS.

If this doesn't help then post your docker-compose or docker run command as well as the output from ls -ln from your NAS.

Solved. I just copy documents, photos etc to docker, but still use MariaDB on host. After copying (moving) NextCloud data files to docker it was necessary to run in docker:

So DB structure of DB was rebuild according to new files location. It taking some time to reindex, creates photos miniature etc. Whole process takes at least 2 hours in my case, but it is acceptable. Migration in on-time procedure.