Hey how’s it going? Around these parts the RPi stuff is hiding, not in a mean way but more like it’s just a community topic that happens to have an odd title and not an official section like the other PMS servers have. I don’t have the answer off the top of my head about USB drives, a Pi, and dockers sorry.
That should get you a working server. I don’t see the need myself for a docker when PMS is going to be the only thing running on the Pi besides maybe a news daemon.
Let me know how it goes.