Add docker package build

[hmacdope]

Branches off #340, so that will have to go in first.

Adds a CI workflow that publishes a docker container to ghcr.io on push to main.

Just a suggestion, if yall don't want or need this feel free to close.

[hmacdope]

Alternatively, could build package on release trigger rather than on main

[hmacdope]

@jthorton in light of our discussion with @j-wags this is probably a bridge too far. Lets proceed with a more minimal docker setup in your PR #340. Closing here.