fix(builder): skip docker rmi if there are no images to remove

Author: mboersma

builder/image/templates/check-repos

@@ -16,8 +16,8 @@ do
     if ! listcontains "{{ join (lsdir "/deis/services/*") " " }}" "$reponame";
     then
         rm -rf "$repo"
-        docker images | grep $appname | awk '{ print $3 }' | xargs docker rmi -f
+        docker images | grep $appname | awk '{ print $3 }' | xargs -r docker rmi -f
         # remove any dangling images left over from the cleanup
-        docker images --filter "dangling=true" | awk '{ print $3 }' | grep -v IMAGE | xargs docker rmi -f
+        docker images --filter "dangling=true" | awk '{ print $3 }' | grep -v IMAGE | xargs -r docker rmi -f
     fi
 done