ci: switch image publishing to docker/github-builder

[crazy-max]

follow-up docker/buildx#3708 (comment)

Description

This PR switches the image build and publish workflow to the Docker GitHub Builder reusable workflow. This keeps the multi-platform image publishing flow while reducing repository-specific GitHub Actions logic.

More info: https://docs.docker.com/build/ci/github-actions/github-builder/

Motivation and Context

This change simplifies the CI pipeline compared to Docker's distributed multi-runner workflow. It removes the need for custom per-platform orchestration, digest artifact handoff, and a separate manifest merge step. It also gives us a more consistent Docker-maintained build path with better provenance and lower maintenance overhead.

How Has This Been Tested?

The workflow configuration was reviewed to confirm that the build arguments, tag strategy, publishing targets, and push behavior remain aligned with the current process. No application runtime code paths were changed in this PR. End-to-end validation will occur in the next development workflow run and the next release publication.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

cc @yubiuser

[crazy-max]

Added pull_request event so workflow can be tested to avoid regressions. We could probably merge build-and-publish.yml with this one as follow-up.

[PromoFaux]

I'll follow this up - I don't recall exactly why I split publish/test out to two distinct workflows, but I think it was just to keep things nice and readable. Less of an issue now...

[rdwebdesign]

Thanks!

[PromoFaux]

End-to-end validation will occur in the next development workflow run and the next release publication.

We can pre-check the development workflow on the experimental branch

https://github.com/pi-hole/docker-pi-hole/actions/runs/23194244321

[PromoFaux]

Checked on experimental branch - looks good to me

[PromoFaux]

Thanks @crazy-max - I go on holiday for one week and come back to a vastly simplified* build yaml... I should go on holiday more often! 😅

* Though I understand the actual process in the background is just as, if not more, complex than what this replaces. I like that it effectively offloads the actual build implementation logic to a Docker-maintained workflow