A somewhat similar permission issue came up here :
https://github.com/pi-hole/docker-pi-hole/issues/330
The fix was a reinstall of container station. QNAP Container station has been hit or miss for several people, I don't know what causes it exactly but they do some special directory / permission setup for docker.
Stock docker is all I have to work with unfortunately so if there are any code fixes that are needed I'll need some direction from community members custom hacking the image (or maybe container station) to see what fixes things.