Automate cross-compiler container image generation

[kallisti5]

I was made aware by a member of the SDL team that docker.io/haiku/cross-compiler existed.
We really need to automate the generation of these (and document them!)

I'm migrating the existing cross-compiler images over to ghcr.io since that's where containers live now.

[kallisti5]

@madebr FYI

[jessicah]

You should be able to piggyback off https://github.com/haiku/haiku-toolchains-ubuntu/tree/master/.github/workflows. There is also my cross-compiler stuff I maintain: https://github.com/jessicah/cross-compiler.

[nielx]

I guess I am the main user of these containers for my rust builds. Sounds good to migrate it over to the github package repository with the rest of our stuff.