Update vendor

vendor/github.com/google/go-containerregistry/pkg/v1/empty/index.go

@@ -0,0 +1,63 @@
+// Copyright 2018 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 empty
+
+import (
+	"encoding/json"
+	"errors"
+
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/partial"
+	"github.com/google/go-containerregistry/pkg/v1/types"
+)
+
+// Index is a singleton empty index, think: FROM scratch.
+var Index = emptyIndex{}
+
+type emptyIndex struct{}
+
+func (i emptyIndex) MediaType() (types.MediaType, error) {
+	return types.OCIImageIndex, nil
+}
+
+func (i emptyIndex) Digest() (v1.Hash, error) {
+	return partial.Digest(i)
+}
+
+func (i emptyIndex) Size() (int64, error) {
+	return partial.Size(i)
+}
+
+func (i emptyIndex) IndexManifest() (*v1.IndexManifest, error) {
+	return base(), nil
+}
+
+func (i emptyIndex) RawManifest() ([]byte, error) {
+	return json.Marshal(base())
+}
+
+func (i emptyIndex) Image(v1.Hash) (v1.Image, error) {
+	return nil, errors.New("empty index")
+}
+
+func (i emptyIndex) ImageIndex(v1.Hash) (v1.ImageIndex, error) {
+	return nil, errors.New("empty index")
+}
+
+func base() *v1.IndexManifest {
+	return &v1.IndexManifest{
+		SchemaVersion: 2,
+	}
+}