fix(Makefile, ci): update readme on hub.docker.io automatically

.travis.yml

@@ -2,4 +2,5 @@ services:
   - docker
 
 script:
-  - make image-push
+  - make push
+  - make update/readme

Makefile

@@ -1,16 +1,58 @@
-# VERSION
-# If no version is specified as a parameter of make, the value latest
-# is taken.
-VERSION:=$(or ${TRAVIS_TAG}, latest)
+# CONTAINER_IMAGE_VERSION
+# Defines the version of the container image which has included the executable
+# binary. This is somethimes different to the variable CONTAINER_IMAGE_VERSION, because the
+# container image version should be latest instead contains the latest git tag
+# and hash sum.
+CONTAINER_IMAGE_VERSION:=$(or ${TRAVIS_TAG}, latest)
+
+# CONTAINER_RUNTIME
+# The CONTAINER_RUNTIME variable will be used to specified the path to a
+# container runtime. This is needed to build the container image.
+CONTAINER_RUNTIME:=$(shell which docker)
 
 # DOCKER_USER
+# It's the username from hub.docker.io. The name in addition with the password
+# is necessary to upload the container image.
 DOCKER_USER:=volkerraschek
 
-image-build:
-	docker build \
-		--tag ${DOCKER_USER}/build-image:${VERSION} \
+# IMAGE_NAME
+# The name of the container image.
+IMAGE_NAME:=build-image
+
+
+# BUILD CONTAINER IMAGE
+# ==============================================================================
+PHONY:=build
+build:
+	${CONTAINER_RUNTIME} build \
+		--file Dockerfile \
+		--no-cache \
+		--tag ${DOCKER_USER}/${IMAGE_NAME}:${CONTAINER_IMAGE_VERSION} \
 		.
 
-image-push: image-build
-	docker login --username ${DOCKER_USER} --password ${DOCKER_PASSWORD}
-	docker push ${DOCKER_USER}/build-image:${VERSION}
+# PUSH CONTAINER IMAGE
+# ==============================================================================
+PHONY+=push
+push: build
+	${CONTAINER_RUNTIME} login docker.io --username ${DOCKER_USER} --password ${DOCKER_PASSWORD}
+	${CONTAINER_RUNTIME} push ${DOCKER_USER}/${IMAGE_NAME}:${CONTAINER_IMAGE_VERSION}
+
+# UPDATE HUB.DOCKER.IO README
+# ==============================================================================
+PHONY+=update/readme
+update/readme:
+	${CONTAINER_RUNTIME} run \
+		--rm \
+		--env DOCKERHUB_USERNAME=${DOCKER_USER} \
+		--env DOCKERHUB_PASSWORD=${DOCKER_PASSWORD} \
+		--env DOCKERHUB_REPOSITORY=${DOCKER_USER}/${IMAGE_NAME} \
+		--env README_FILEPATH=./README.md \
+		--volume $(shell pwd):/workspace:ro \
+		--workdir=/workspace \
+		peterevans/dockerhub-description:latest
+
+# PHONY
+# ==============================================================================
+# Declare the contents of the PHONY variable as phony.  We keep that information
+# in a variable so we can use it in if_changed.
+.PHONY: ${PHONY}