add build-arg ARCH for building multi-arch images, e.g.

ARG ARCH
FROM k8s.gcr.io/build-image/debian-base-${ARCH}:v2.1.3

build.make

@@ -149,6 +149,7 @@ $(CMDS:%=push-multiarch-%): push-multiarch-%: check-pull-base-ref build-%
 				--platform=$$os/$$arch \
 				--file $$(eval echo \$${dockerfile_$$os}) \
 				--build-arg binary=./bin/$*$$suffix \
+				--build-arg ARCH=$$arch \
 				--label revision=$(REV) \
 				.; \
 		done; \