Have you got to grips with mapping storage locations from your host (QNAP) to the Docker container at all? You’ll need all of your persistent data mapped to storage outside of the container on the NAS. The Docker container provides four mapping “hints”:
/app for application data
/data for Roon’s database, best mapped to SSD storage if possible
`/music’ for your music library
`/backup’ for Roon backups
The NAS path to your music share needs mapping to /music on the Docker container. This thread may help: