Add make rule to build amd64 and armv7 docker images with docker buildx

Makefile

-FULLTAG=alpinebased:grafana-backup
+
+DOCKER_REPO ?= ysde
+DOCKER_NAME := grafana-backup
+DOCKER_TAG ?= latest
+PLATFORMS ?= linux/amd64,linux/arm/v7
+
+FULLTAG = $(DOCKER_REPO)/$(DOCKER_NAME):$(DOCKER_TAG)
+
 DOCKERFILE=Dockerfile
+
 all: build
 
 build:
 	docker build -t $(FULLTAG) -f $(DOCKERFILE) .
+
 push: build
 	docker push $(FULLTAG)
+
+
+buildx_and_push:
+	docker buildx build \
+		--output type=image,name=$(DOCKER_REPO)/$(DOCKER_NAME),push=true \
+	 	--platform linux/amd64,linux/arm/v7 \
+		--tag $(FULLTAG) \
+	 	--file $(DOCKERFILE) .