Persistent volume caching for base images (#383)

* comments

* initial commit for persisent volume caching

* cache warmer works

* general cleanup

* adding some debugging

* adding missing files

* Fixing up cache retrieval and cleanup

* fix tests

* removing auth since we only cache public images

* simplifying the caching logic

* fixing logic

* adding volume cache to integration tests. remove auth from cache warmer image.

* add building warmer to integration-test

* move sample yaml files to examples dir

* small test fix

cmd/executor/cmd/root.go

@@ -99,6 +99,7 @@ func addKanikoOptionsFlags(cmd *cobra.Command) {
 	RootCmd.PersistentFlags().StringVarP(&opts.Target, "target", "", "", "Set the target build stage to build")
 	RootCmd.PersistentFlags().BoolVarP(&opts.NoPush, "no-push", "", false, "Do not push the image to the registry")
 	RootCmd.PersistentFlags().StringVarP(&opts.CacheRepo, "cache-repo", "", "", "Specify a repository to use as a cache, otherwise one will be inferred from the destination provided")
+	RootCmd.PersistentFlags().StringVarP(&opts.CacheDir, "cache-dir", "", "/cache", "Specify a local directory to use as a cache.")
 	RootCmd.PersistentFlags().BoolVarP(&opts.Cache, "cache", "", false, "Use cache when building image")
 	RootCmd.PersistentFlags().BoolVarP(&opts.Cleanup, "cleanup", "", false, "Clean the filesystem at the end")
 }