WIP: [ci] move CI container images to GHCR, workflows to this repo

[jameslamb]

Fixes #5596
Fixes #7011

Moves Dockerfiles and CI pipelines for the container images used in Linux CI into this repo. New pipelines here will publish CI images to GitHub's container registry.

Other changes:

• upgrades from PoCL 1.8 to 7.1 (no more building clang from source 😁)
• moves aarch64 Linux wheels up to GLIBC 2.28 (manylinux_2_28)

Notes for Reviewers

Post-merge cleanup

After this is merged and once we feel like it's working well, all of the following could be cleaned up:

• owned by @guolinke :
  • https://github.com/guolinke/lightgbm-ci-docker archived / deleted
  • https://hub.docker.com/repository/docker/lightgbm/vsts-agent/general
• owned by @shiyu1994 (or maybe @letmaik if it's inside Microsoft)
  • lightgbm.azurecr.io/vsts-agent image repo
• other:
  • DockerHub webhook(s) in this repo