Set labels on the Docker image (#57)

To improve traceability of our Docker image and add other useful
metadata let's set a few OCI labels, plus a Datadog label to make
life easier for users of Datadog without impacting others.

There no foolproof way of detecting the repository we're building
from so the repository URL is unfortunately hardcoded.

Makefile

@@ -59,7 +59,7 @@ stop:
 # Build a docker using the production Dockerfile
 .PHONY: docker
 docker:
-	docker build -t $(DOCKER_REGISTRY)/$(DEPLOY):$(RELEASE_VERSION) -f ./deploy/$(DEPLOY)/Dockerfile .
+	docker build --build-arg revision=$(RELEASE_VERSION) -t $(DOCKER_REGISTRY)/$(DEPLOY):$(RELEASE_VERSION) -f ./deploy/$(DEPLOY)/Dockerfile .
 
 .PHONY: push
 push:

deploy/goer/Dockerfile

@@ -9,3 +9,9 @@ RUN make build
 FROM alpine:3.13.5
 ENTRYPOINT ["/app/goer"]
 COPY --from=build /tmp/goer/bin/goer /app/goer
+
+ARG revision
+LABEL com.datadoghq.tags.version="${revision}" \
+      org.opencontainers.image.licenses="Apache-2.0" \
+      org.opencontainers.image.revision="${revision}" \
+      org.opencontainers.image.source="https://github.com/eiffel-community/eiffel-goer"