refactor makefile: rename variables (including removal of "DOCKER_" prefixes)

https://github.com/fphammerle/docker-onion-service/commit/4c673eef480af3bb9498c0364a55dc3d8f8a5afc
https://github.com/fphammerle/docker-onion-service/commit/a3e89529767a7eb52c4bce52277715d388f6935a
https://github.com/fphammerle/docker-onion-service/commit/571aa8c6351c474dffd0603e9b9ec1ec3cd4c140

Makefile

@@ -1,20 +1,22 @@
 # sync with https://github.com/fphammerle/switchbot-mqtt/blob/master/Makefile
 
-DOCKER_IMAGE_NAME := fphammerle/systemctl-mqtt
-DOCKER_TAG_VERSION := $(shell git describe --match=v* --dirty | sed -e 's/^v//')
-ARCH := $(shell arch)
-DOCKER_TAG_ARCH_SUFFIX_aarch64 := arm64
-DOCKER_TAG_ARCH_SUFFIX_armv6l := armv6
-DOCKER_TAG_ARCH_SUFFIX_armv7l := armv7
-DOCKER_TAG_ARCH_SUFFIX_x86_64 := amd64
-DOCKER_TAG_ARCH_SUFFIX = ${DOCKER_TAG_ARCH_SUFFIX_${ARCH}}
-DOCKER_TAG = ${DOCKER_TAG_VERSION}-${DOCKER_TAG_ARCH_SUFFIX}
+IMAGE_NAME = docker.io/fphammerle/systemctl-mqtt
+BUILD_VERSION = $(shell git describe --match=v* --dirty | sed -e 's/^v//')
+ARCH = $(shell arch)
+# architecture[arm_variant]
+# https://github.com/opencontainers/image-spec/blob/v1.0.1/image-index.md#image-index-property-descriptions
+IMAGE_TAG_ARCH_aarch64 = arm64
+IMAGE_TAG_ARCH_armv6l = armv6
+IMAGE_TAG_ARCH_armv7l = armv7
+IMAGE_TAG_ARCH_x86_64 = amd64
+IMAGE_TAG_ARCH = ${IMAGE_TAG_ARCH_${ARCH}}
+IMAGE_TAG = ${BUILD_VERSION}-${IMAGE_TAG_ARCH}
 
 .PHONY: docker-build docker-push
 
 docker-build:
-	sudo docker build -t "${DOCKER_IMAGE_NAME}:${DOCKER_TAG}" .
+	sudo docker build -t "${IMAGE_NAME}:${IMAGE_TAG}" .
 
 docker-push: docker-build
-	sudo docker push "${DOCKER_IMAGE_NAME}:${DOCKER_TAG}"
-	@echo git tag --sign --message '$(shell sudo docker image inspect --format '{{join .RepoDigests "\n"}}' "${DOCKER_IMAGE_NAME}:${DOCKER_TAG}")' docker/${DOCKER_TAG} $(shell git rev-parse HEAD)
+	sudo docker push "${IMAGE_NAME}:${IMAGE_TAG}"
+	@echo git tag --sign --message '$(shell sudo docker image inspect --format '{{join .RepoDigests "\n"}}' "${IMAGE_NAME}:${IMAGE_TAG}")' docker/${IMAGE_TAG} $(shell git rev-parse HEAD)