OK, resolved it by myself. I used the restore-script which you can execute with Docker before the openHAB Service starts:
On the Docker host I created /opt/openhab/cont-init.d/restore.sh which contains one line:
echo y | /bin/bash /openhab/runtime/bin/restore /openhab/backups/backup.zip
The backup.zipwas created with a simplye openhab-cli backupon the source system. No --fullneeded. Then I mounted the backup file and the script into the container like this:
volumes:
(...)
- $OPENHAB_MAIN/cont-init.d:/etc/cont-init.d
- $OPENHAB_MAIN/backups:/openhab/backups
And then everything works like a charm. Finally. 