Remove the local manifest list after push

Manifests seem sticky in docker, so let's try to purge so if
we have re-push a fresh set of containers (with same version number as
before) during testing, the manifests are created fresh.

Change-Id: I41c010c08bd50b68ff6973a4ae1e004824fab178

test/images/image-util.sh

@@ -112,7 +112,7 @@ push() {
   for arch in ${archs}; do
     docker manifest annotate --arch ${arch} ${REGISTRY}/${IMAGE}:${TAG} ${REGISTRY}/${IMAGE}-${arch}:${TAG}
   done
-  docker manifest push ${REGISTRY}/${IMAGE}:${TAG}
+  docker manifest push --purge ${REGISTRY}/${IMAGE}:${TAG}
 }
 
 # This function is for building the go code