Issue Backup on Docker Container fails

[Bender_]

I run a mariadb Container with bind mount located at my subdomain (/var/www/vhosts/xyz.de/cloud.xyz.de) that i use for my case.

pleskbackup runs into error backuping certain files, from the log:

[2021-05-24 03:13:07.167|44695] INFO: cloud.xyz.de/nextcloud-db/mysql: Cannot open: Permission denied
cloud.xyz.de/nextcloud-db/mysql/column_stats.MAD: Cannot stat: Permission denied
cloud.xyz.de/nextcloud-db/mysql/column_stats.MAI: Cannot stat: Permission denied
cloud.xyz.de/nextcloud-db/mysql/column_stats.frm: Cannot stat: Permission denied
cloud.xyz.de/nextcloud-db/mysql/columns_priv.MAD: Cannot stat: Permission denied
cloud.xyz.de/nextcloud-db/mysql/columns_priv.MAI: Cannot stat: Permission denied
cloud.xyz.de/nextcloud-db/mysql/columns_priv.frm: Cannot stat: Permission denied

An extract of the file persmission as created by docker:

root@xyz:/var/www/vhosts/xyz.de/cloud.xyz.de/nextcloud-db/mysql# ls -la
total 10816
drwx------ 2 psaadm systemd-journal    4096 Mai 24 15:41 .
drwxr-xr-x 5 psaadm root               4096 Mai 24 15:41 ..
-rw-rw---- 1 psaadm systemd-journal    2108 Mai 24 15:41 columns_priv.frm
-rw-rw---- 1 psaadm systemd-journal    8192 Mai 24 15:41 columns_priv.MAD
-rw-rw---- 1 psaadm systemd-journal    8192 Mai 24 15:41 columns_priv.MAI

My idea was to change the group from "systemd-journal" to "docker" and add the user that runs the backups to the group "docker". I tried that with the user "backup" without success. Of course giving permissions to other works too, but that cant be the goal.

What user do i have to add to the group docker? Under what context are the backups running?

htop displayed only root.