Ah, and I just realized that your QNAP (and most other POSIX systems!) use tmpfs for /tmp.
This means all of /tmp is wiped on reboot, so /tmp/photostructure-tmp won’t exist.
Docker doesn’t, as far as I know, have a switch to auto-mkdir any missing bind mount volumes, so unless something runs mkdir /tmp/photostructure-tmp before docker starts, the container will fail to spin up.
The next release of PhotoStructure will support using /tmp directly, and create a subdirectory with a umask of 077 to avoid this issue. I’ll update instructions when that release is stable.