Still support building default in docker

Since the docker image is always running Linux amd64, we need to
pass the actual local target to build (cross-compile) minikube for.

Makefile

@@ -143,7 +143,11 @@ endif
 
 
 out/minikube$(IS_EXE): $(SOURCE_GENERATED) $(SOURCE_FILES) go.mod
+ifeq ($(MINIKUBE_BUILD_IN_DOCKER),y)
+	$(call DOCKER,$(BUILD_IMAGE),GOOS=$(GOOS) GOARCH=$(GOARCH) /usr/bin/make $@)
+else
 	go build -tags "$(MINIKUBE_BUILD_TAGS)" -ldflags="$(MINIKUBE_LDFLAGS)" -o $@ k8s.io/minikube/cmd/minikube
+endif
 
 out/minikube-windows-amd64.exe: out/minikube-windows-amd64
 	cp $< $@