Inject Git source commit metadata into the image

These values are useful for looking up the state of code when examining
images at a particular tag. When setting LABEL values, we can easily
pull them out of an image through inspection:

    docker inspect --format '{{.Config.Labels}}' tiddlywiki

The default value of `unknown` is meant to catch the accidental case
where an image was pushed manually. The naming of the `SOURCE_COMMIT`
variable itself comes from the convention used by Docker Cloud [1] (but
might still be undocumented). Labels seemed more appropriate than using
ENV variables.

[1] docker/hub-feedback#600 (comment)

5/Dockerfile

@@ -1,8 +1,12 @@
 FROM node:alpine
-LABEL maintainer="Aaron Bull Schaefer <aaron@elasticdog.com>"
 
 ENV TIDDLYWIKI_VERSION=5.1.17
 
+ARG SOURCE_COMMIT
+LABEL maintainer="Aaron Bull Schaefer <aaron@elasticdog.com>"
+LABEL source="https://github.com/elasticdog/tiddlywiki-docker"
+LABEL source-commit="${SOURCE_COMMIT:-unknown}"
+
 # https://github.com/nodejs/docker-node/blob/master/docs/BestPractices.md#handling-kernel-signals
 RUN apk add --no-cache tini
 RUN npm install -g tiddlywiki@${TIDDLYWIKI_VERSION}

Makefile

@@ -4,8 +4,9 @@ DOCKER_HUB_REPOSITORY ?= elasticdog
 all: build
 
 .PHONY: build
+build: SOURCE_COMMIT := $(shell git rev-parse HEAD)
 build:
-	docker build --tag tiddlywiki .
+	docker build --build-arg "SOURCE_COMMIT=${SOURCE_COMMIT}" --tag tiddlywiki .
 
 .PHONY: test
 test:
@@ -16,7 +17,9 @@ tag: PATCH_VERSION := $(shell docker run -it --rm tiddlywiki --version | sed 's/
 tag: MINOR_VERSION := $(shell echo "${PATCH_VERSION}" | awk -F. '{ print $$1"."$$2 }')
 tag: MAJOR_VERSION := $(shell echo "${PATCH_VERSION}" | awk -F. '{ print $$1 }')
 tag:
+	# unique tag
 	if [ "$$CI" = true ]; then docker tag tiddlywiki "${DOCKER_HUB_REPOSITORY}/tiddlywiki:$$(date +%Y%m%d).$${CIRCLE_BUILD_NUM}"; fi
+	# stable tags
 	docker tag tiddlywiki "${DOCKER_HUB_REPOSITORY}/tiddlywiki:latest"
 	docker tag tiddlywiki "${DOCKER_HUB_REPOSITORY}/tiddlywiki:${PATCH_VERSION}"
 	docker tag tiddlywiki "${DOCKER_HUB_REPOSITORY}/tiddlywiki:${MINOR_VERSION}"