We build multiarch linux/amd64 and linux/arm64, because MacOS runner can't
build docker images and even if it could that'd be linux/amd64 while we want
linux/arm64 for Apple CPUs.
Unfortunately, given the way GitHub workflows work we can't avoid using a
Makefile helper, there is no easy way to set variables conditionally and/or
use some logic to affect their contents.
We reintroduce build_image_wsc as well here because Windows images can't be
built with buildx using GitHub runners.
Unfortunately, the default MacOS runner produces amd64 binaries, therefore we
need to set GOARCH appropriately. At the same time, docker image will be
linux/arm64, so we can build it as well.
