Add insecure-registry and tls-skip-verify-registry flags (#537)

pkg/executor/build.go

@@ -145,6 +145,7 @@ func (s *stageBuilder) optimize(compositeKey CompositeCache, cfg v1.Config, cmds
 		if command.ShouldCacheOutput() {
 			img, err := layerCache.RetrieveLayer(ck)
 			if err != nil {
+				logrus.Debugf("Failed to retrieve layer: %s", err)
 				logrus.Infof("No cached layer found for cmd %s", command.String())
 				logrus.Debugf("Key missing was: %s", compositeKey.Key())
 				break