[WIP] Dockerfile for OctoPrint on Raspberry Pi

Builds and runs correctly using the docker-enabled Arch Linux image, [documented here.](https://github.com/MissionSt/printer-client/blob/2886b8555f8a9574abd6bfef9c24b2c08dd01f2a/docker/README.md)

See comments for some TODOs regarding unnecessary superuser privileges.