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.

extras/example/echo/Dockerfile

@@ -1,8 +1,14 @@
 FROM tatsushid/tinycore-python:2.7
-LABEL maintainer="Weaveworks Inc <help@weave.works>"
 WORKDIR /home/weave
 ADD requirements.txt /home/weave/
 RUN pip install -r /home/weave/requirements.txt
 ADD echo.py /home/weave/
 EXPOSE 5000
 ENTRYPOINT ["python", "/home/weave/echo.py"]
+
+ARG revision
+LABEL maintainer="Weaveworks <help@weave.works>" \
+      org.opencontainers.image.title="example-echo" \
+      org.opencontainers.image.source="https://github.com/weaveworks/scope/tree/master/extras/example/echo" \
+      org.opencontainers.image.revision="${revision}" \
+      org.opencontainers.image.vendor="Weaveworks"