fix: correctly set image labels build time

hooks/build

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+# as per https://github.com/rossf7/label-schema-automated-build
+
+# $IMAGE_NAME var is injected into the build so the tag is correct.
+
+echo "Build hook running"
+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 .