I don’t see anything mapped to /backups in your container. That may mean Duplicati is trying to write backup files into the container itself, which would be bad.
But beyond that, note that the linuxserver duplicati image runs Duplicati as a limited user by default. So you’ll need to make sure it has read and write access were appropriate. (The official duplicati docker image runs Duplicati as root by default, so it has access to everything.)