workflows/docker: build and publish ARM64 Docker images

[ZhongRuoyu]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This modifies the Docker workflow to first build the images natively on x86_64 and ARM64 runners, push them by digest to the registry, and then merge the manifest lists to form a tagged multi-platform image. This allows e.g. docker run homebrew/brew to work on both platforms.

Ref: https://docs.docker.com/build/ci/github-actions/multi-platform/

[carlocab]

LGTM. We can probably skip 18.04 on ARM, since we want to 🔥 that image anyway. See #19190.

Required CI checks need updating too by an org owner (or admin?).

[Bo98]

Perhaps only want to push on master tags initially too? And then do release tags after some bottle coverage?