build: wip use buildx to build multi-platform docker images

[holiman]

This is a work in progress of a buildx-based docker image creator. Our current builder is very complicated:

• Use multiple builders, on different platforms (arm vs amd).
• They build and push their own native platform
• One of them (random which, decided by a race) generates a manifest image, which basically says "stable == stable-amd64 and/or stable/amd64".

With docker buildx, we can build all archs in one go.

If I run this on master (to disable the maybeSkipArchive), and set DryRun to true in util.go and execute, these are the actions it would have attempted:

[user@work go-ethereum]$ go run build/ci.go dockerx  -manifest amd64,arm64 -upload ethereum/client-go 
>>> docker buildx --build-arg COMMIT=283be238172c8ae3a05f97e2f3c159f5c2319e07 --build-arg VERSION=1.14.11-unstable --build-arg BUILDNUM= --tag ethereum/client-go:latest-amd64 --platform amd64 --load --push --file Dockerfile .
>>> docker buildx --build-arg COMMIT=283be238172c8ae3a05f97e2f3c159f5c2319e07 --build-arg VERSION=1.14.11-unstable --build-arg BUILDNUM= --tag ethereum/client-go:latest-arm64 --platform arm64 --load --push --file Dockerfile .
>>> docker buildx imagetools create --tag ethereum/client-go:latest ethereum/client-go:latest-amd64 ethereum/client-go:latest-arm64
>>> docker buildx --build-arg COMMIT=283be238172c8ae3a05f97e2f3c159f5c2319e07 --build-arg VERSION=1.14.11-unstable --build-arg BUILDNUM= --tag ethereum/client-go:alltools-latest-amd64 --platform amd64 --load --push --file Dockerfile.alltools .
>>> docker buildx --build-arg COMMIT=283be238172c8ae3a05f97e2f3c159f5c2319e07 --build-arg VERSION=1.14.11-unstable --build-arg BUILDNUM= --tag ethereum/client-go:alltools-latest-arm64 --platform arm64 --load --push --file Dockerfile.alltools .
>>> docker buildx imagetools create --tag ethereum/client-go:alltools-latest ethereum/client-go:alltools-latest-amd64 ethereum/client-go:alltools-latest-arm64

Now, this is a bit gnarly to experiment with on travis, and I also don't want to bork the existing images. So maybe I'll just push to a new tag, e.g., instead of ethereum/client-go, we could use ethereum/client-gox until it works?

[holiman]

replaced by #30530