Contributes to #7011

7109 was my first attempt to move the contents of https://github.com/lightgbm-org/lightgbm-ci-docker into this repo, and to publish images to GHCR instead of DockerHub.

It got stuck on some build issues related to PoCL, Boost, and OpenCL. I'll fix that in a future PR.

This PR pulls out some of the main setup of self-publishing CI images.

• introduces the machinery for building images in CI and pushing them to GHCR (as "packages" in this repo)


• adds a build of a ppc64le image, to help with https://github.com/lightgbm-org/LightGBM/pull/7373#discussion_r3667460806 / https://github.com/lightgbm-org/LightGBM/issues/7262
  

Notes for Reviewers

How I tested this

Triggered workflow runs on this branch using the approach described in the MAINTAINING docs (added in this PR).

Applied these images over in the PR adding a new job building ppc64le Python wheels and saw it work! https://github.com/lightgbm-org/LightGBM/pull/7373#discussion_r3853847556

You can also go to https://github.com/orgs/lightgbm-org/packages?repo_name=LightGBM to see the tags.