For anyone getting a "permissions denied" error when its trying to generate the thumbnail OR getting a "no such file or directory" you need to change the permissions of the photoview folder that is created inside of your appdata. I'm not exposing this docker publicly and I would strongly recommend you do this the right way (actually setting the correct permissions for the correct user).
I changed my permissions on the photoview folder to rwxr-xrwx instead (0756 instead of 0755)
Restarted photoview and my thumbnails were finally being generated. The default configuration is actually correct, its the permissions that need to be updated. Assuming you're not having issues with your database. I am using MariaDB, haven't had any issues - also didn't try any other database.
If you're getting a "file or directory not found" error, you probably had changed the `/app/cache` path at the bottom of the container. Set that back to the default unless you understand what you're doing.
edit: Checkout this github issue to see if you're having the same problem. https://github.com/photoview/photoview/issues/946