Refactoring PMS container

[errordeveloper]

The official container works, but it's running multiple processes under an init daemon, it also downloads binaries at runtime.

We should find way to minimise the complexity of PMS container, potentially break it up into multiple containers.

Here are some pointers to prior art:

• https://wiki.archlinux.org/index.php/plex
• https://github.com/spritsail/plex-media-server (uses Debian with multi-stage builds, but entrypoint script is too complicated)
• https://github.com/oskarirauta/alpine-plex (for of official image, uses Alpine but doesn't do anything different)