tests: build multiplatform

[idnandre]

add test build and bake multiplatform (--platform), to improve test coverage based on #1857

[thaJeztah]

@crazy-max how does this check detect whether the docker daemon supports multi-arch? Having a quick look; isMobyWorker and isMobyContainerdSnapWorker look identical, but if docker is running with the containerd image-store enabled, it supports multi-arch?

buildx/tests/integration.go

Lines 97 to 105 in 56cb197

	func isMobyWorker(sb integration.Sandbox) bool {
	name, hasFeature := driverName(sb.Name())
	return name == "docker" && !hasFeature
	}
	func isMobyContainerdSnapWorker(sb integration.Sandbox) bool {
	name, hasFeature := driverName(sb.Name())
	return name == "docker" && hasFeature
	}

Should we have a supportsMultiArch check instead?

[thaJeztah]

OH! Nevermind; hasFeature vs !hasFeature 🙈 I missed the ! 🙈 😂

Still, maybe a supportsMultiArch or hasMultiarchSupport could make sense (more descriptive?)

[crazy-max]

This is not just multiarch but also cache and oci exporters that are supported with containerd snap:

buildx/driver/docker/driver.go

Lines 92 to 95 in aa35c95

	driver.OCIExporter: useContainerdSnapshotter,
	driver.DockerExporter: useContainerdSnapshotter,
	driver.CacheExport: useContainerdSnapshotter,
	driver.MultiPlatform: useContainerdSnapshotter,

[crazy-max]

LGTM thanks!