add xc-prod-image for cross compiling vault-k8s image in release pipe…

…line (hashicorp#221)

Makefile

@@ -1,16 +1,17 @@
 REGISTRY_NAME?=docker.io/hashicorp
 IMAGE_NAME=vault-k8s
 VERSION?=0.8.0
-IMAGE_TAG=$(REGISTRY_NAME)/$(IMAGE_NAME):$(VERSION)
+IMAGE_TAG?=$(REGISTRY_NAME)/$(IMAGE_NAME):$(VERSION)
 PUBLISH_LOCATION?=https://releases.hashicorp.com
 DOCKER_DIR=./build/docker
 BUILD_DIR=.build
 GOOS?=linux
 GOARCH?=amd64
 BIN_NAME=$(IMAGE_NAME)_$(GOOS)_$(GOARCH)_$(VERSION)
 GOFMT_FILES?=$$(find . -name '*.go' | grep -v vendor)
+XC_PUBLISH?=
 
-.PHONY: all test build image clean 
+.PHONY: all test build image clean
 all: build
 
 build:
@@ -33,6 +34,16 @@ prod-ubi-image:
     --build-arg LOCATION=$(PUBLISH_LOCATION) \
     -f $(DOCKER_DIR)/Release.ubi.dockerfile .
 
+# This target is used in CI to cross compile vault-k8s for 4 different architectures
+# and publish (when XC_PUBLISH="--push") using docker buildx
+xc-prod-image:
+	docker buildx build --platform linux/amd64,linux/arm64,linux/386,linux/arm/v6 \
+	--build-arg VERSION="${RELEASE_VERSION}" \
+	--build-arg LOCATION="${PUBLISH_LOCATION}" \
+	$(XC_PUBLISH) \
+	-t $(IMAGE_TAG) \
+	-f ${DOCKER_DIR}/Release.dockerfile .
+
 clean:
 	-rm -rf $(BUILD_DIR)