CI: create a docker image for arm64 as well

[shtouff]

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[batfish-bot]

This change is 

[shtouff]

according to https://docs.docker.com/engine/reference/commandline/buildx_build/#output, the docker output type for buildx is incompatible with multi-platform images. Therefore, I choose to duplicate build and upload tasks for both images.

[dhalperi]

I've asked @sfraint to look at this; he set it up. But I wonder if https://docs.docker.com/build/ci/github-actions/multi-platform/ doesn't imply that we should be able to do two images in one?

[shtouff]

I've asked @sfraint to look at this; he set it up. But I wonder if https://docs.docker.com/build/ci/github-actions/multi-platform/ doesn't imply that we should be able to do two images in one?

you were right, I'm reactivating this PR, tryin to build a single, multi-platform image, using the OCI format.

However, I don't know how to reproduce github actions on my laptop, so it's gonna be a lot a try-and-error, each of them requiring an approval for the workflow. Sorry for that noise.