Docker build hook to add label info

hooks/build

+#!/bin/bash
+
+# $IMAGE_NAME var is injected into the build so the tag is correct. 
+docker build --build-arg BUILD_DATE=`date -u +"%Y-%m-%dT%H:%M:%SZ"` \
+             --build-arg VCS_REF=`git rev-parse --short HEAD` \
+             -t $IMAGE_NAME .