[WIP] Docker multi-arch arm7/8

[mburns]

connected to #7541

What

Docker has a (still experimental) buildx plugin to make multi-arch docker images simply.

Why

Using buildx lets us leverage a single Dockerfile, rather having one per-architecture.

Adapted from https://www.docker.com/blog/multi-arch-build-what-about-circleci/

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[mburns]

failed to solve: rpc error: code = Unknown desc = failed to load LLB: runtime execution on platform linux/arm64 not supported

[hsanjuan]

@mburns this looked interesting, any context on why it was discarded?

[mburns]

It is worth reviving, I just didn't want this attempt sitting abandoned when I got distracted with other things.