Build brew docker image for x86 and arm64

[HeroCC]

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When opening the brew repository with VSCode on am M1 Mac, it prompted me to reopen the window in a devcontainer. I agreed, then that devcontainer failed to startup, saying there was no such image. Attempting to pull manually resulted in this:

❯ docker pull ghcr.io/homebrew/brew:latest
latest: Pulling from homebrew/brew
no matching manifest for linux/arm64/v8 in the manifest list entries

This PR will build the linux/arm64 image in parallel with the amd64 image, allowing arm-based macs (and any other arm64-based system) to use the brew container.

[HeroCC]

Looks like the https://github.com/docker/setup-qemu-action is not whitelisted. Could someone add that to the repository settings? It is created and published by the official Docker team / namespace.

https://github.com/Homebrew/brew/actions/runs/11167051891

[MikeMcQuaid]

This PR will build the linux/arm64 image in parallel with the amd64 image, allowing arm-based macs (and any other arm64-based system) to use the brew container.

Homebrew does not support linux/arm64 so we shouldn't be building this image either, sorry.

[MikeMcQuaid]

When opening the brew repository with VSCode on am M1 Mac, it prompted me to reopen the window in a devcontainer.

This devcontainer exists purely for GitHub Codespaces. If there's a way to disable this alert: we should do that, instead.

[Bo98]

❯ docker pull ghcr.io/homebrew/brew:latest
latest: Pulling from homebrew/brew
no matching manifest for linux/arm64/v8 in the manifest list entries

Adding arm64 will require further work such as building arm64 bottles for everything. The best solution for now is to supply --platform linux/amd64 to docker pull so that it uses Docker's in-built emulation (or Rosetta).

This used to happen by default but this unfortunately regressed when we moved to the Docker GitHub Actions, probably because of the way attestations work.