Fix: git commit id labels on images

1) Use OCI Image Specs for labels instead of custom 'commit-id=xxxxx'
   or legacy "Label Schema"
2) Fix missing git commit id labels on images (.revision)
3) Add human-readable title (.title) of the image, URL (.url), and
   a few other properties (annotations) according to the latest Specs

Change-Id: I63c39512ad04f36f63ddb27f0cfd5ef34b7d3467

Dockerfile

@@ -1,5 +1,12 @@
 FROM ubuntu:16.04
 
+LABEL org.opencontainers.image.authors='airship-discuss@lists.airshipit.org, irc://#airshipit@freenode'
+LABEL org.opencontainers.image.url='https://airshipit.org'
+LABEL org.opencontainers.image.documentation='https://airship-berth.readthedocs.org'
+LABEL org.opencontainers.image.source='https://git.openstack.org/openstack/airship-berth'
+LABEL org.opencontainers.image.vendor='The Airship Authors'
+LABEL org.opencontainers.image.licenses='Apache-2.0'
+
 RUN apt-get update && apt-get install -y qemu-kvm dnsmasq bridge-utils mkisofs curl jq wget iptables
 RUN apt-get clean
 RUN rm -f /var/lib/apt/lists/* || true