Combine multi-tag builds into one buildx command using multiple --tag options

[jgriff]

Currently, we build (and push) the image twice when tagging a release with both the full tag (ie 1.2.3) and the major/minor moving tag (ie 1.2).

buildx now (or always?) supports multiple --tag options so we can tag and push from a single build.

We can change this:

	@docker buildx build --builder k8s-resource-builder --platform linux/amd64,linux/arm64/v8 --push --tag ${IMAGE}:$(shell echo ${TAG} | rev | cut -d '.' -f2- | rev ) .
	@docker buildx build --builder k8s-resource-builder --platform linux/amd64,linux/arm64/v8 --push --tag ${IMAGE}:${TAG} .

to this:

	@docker buildx build --builder k8s-resource-builder --platform linux/amd64,linux/arm64/v8 --push --tag ${IMAGE}:${TAG} --tag ${IMAGE}:$(shell echo ${TAG} | rev | cut -d '.' -f2- | rev ) .