thumbnails directory is not owned by www-data

[ppmt]

And because of this the generation of thumbnails for the albums is failing reverting back to the default icons

Forcing the directory to be owned by www-data fixed the issue? I guess it should be added in the dockerfile as well

[ppmt]

I tried to add the chown command in the dockerfile to try to change the ownership but it would always fail

In the end I fix it by modifying the docker_entrypoint.sh file to add the command at the end. Right before the last line (exec "$@") I added:

chown -h www-data: /thumbnails/

And now it works. The thumbnails are generated.

[sebcworks]

This is a little late to answer, but this problem is surely linked to the fact that your "source" thumbnails folder (the one from your host) doesn't have the same uid as www-data inside the container.

The best way to use data with docker is to use volume, this will avoid this kind of problem.