Add cache example (#522)

pkg/v1/cache/example_test.go

@@ -0,0 +1,46 @@
+// Copyright 2019 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cache_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+
+	"github.com/google/go-containerregistry/pkg/v1/cache"
+	"github.com/google/go-containerregistry/pkg/v1/random"
+)
+
+func ExampleImage() {
+	img, err := random.Image(1024*1024, 3)
+	if err != nil {
+		log.Fatal(err)
+	}
+	dir, err := ioutil.TempDir("", "")
+	if err != nil {
+		log.Fatal(err)
+	}
+	fs := cache.NewFilesystemCache(dir)
+
+	// cached will cache layers from img using the fs cache
+	cached := cache.Image(img, fs)
+
+	// Use cached as you would use img.
+	digest, err := cached.Digest()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(digest)
+}