Add org.opencontainers.image.* labels to Dockerfiles

- This should ultimately help for image-to-code back references.
- `org.label-schema.*` labels are now deprecated, in favour of `org.opencontainers.image.*` labels.
  See also: https://github.com/opencontainers/image-spec/blob/master/annotations.md#back-compatibility-with-label-schema
- Git revision (`git rev-parse HEAD`) is now injected at `docker build` time.

docker/Dockerfile.cloud-agent

@@ -1,9 +1,15 @@
 FROM alpine:3.5
-LABEL maintainer="Weaveworks Inc <help@weave.works>"
-LABEL works.weave.role=system
 WORKDIR /home/weave
 RUN apk add --update bash conntrack-tools iproute2 util-linux curl && \
 	rm -rf /var/cache/apk/*
 ADD ./weave ./weaveutil /usr/bin/
 COPY ./scope /home/weave/
 ENTRYPOINT ["/home/weave/scope", "--mode=probe", "--no-app", "--probe.docker=true"]
+
+ARG revision
+LABEL works.weave.role="system" \
+      maintainer="Weaveworks <help@weave.works>" \
+      org.opencontainers.image.title="cloud-agent" \
+      org.opencontainers.image.source="https://github.com/weaveworks/scope" \
+      org.opencontainers.image.revision="${revision}" \
+      org.opencontainers.image.vendor="Weaveworks"