Update vendor

vendor/github.com/google/go-containerregistry/pkg/authn/multikeychain.go

@@ -0,0 +1,41 @@
+// 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 authn
+
+type multiKeychain struct {
+	keychains []Keychain
+}
+
+// Assert that our multi-keychain implements Keychain.
+var _ (Keychain) = (*multiKeychain)(nil)
+
+// NewMultiKeychain composes a list of keychains into one new keychain.
+func NewMultiKeychain(kcs ...Keychain) Keychain {
+	return &multiKeychain{keychains: kcs}
+}
+
+// Resolve implements Keychain.
+func (mk *multiKeychain) Resolve(target Resource) (Authenticator, error) {
+	for _, kc := range mk.keychains {
+		auth, err := kc.Resolve(target)
+		if err != nil {
+			return nil, err
+		}
+		if auth != Anonymous {
+			return auth, nil
+		}
+	}
+	return Anonymous, nil
+}