Update image export to support Docker format

Add manifest.json file which is used by Docker
to import images.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

images/annotations.go

@@ -0,0 +1,28 @@
+/*
+   Copyright The containerd Authors.
+
+   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 images
+
+const (
+	// AnnotationImageName is an annotation on a Descriptor in an index.json
+	// containing the `Name` value as used by an `Image` struct
+	AnnotationImageName = "io.containerd.image.name"
+
+	// AnnotationImageNamePrefix is used the same way as AnnotationImageName
+	// but may be used to refer to additional names in the annotation map
+	// using user-defined suffixes (i.e. "extra.1")
+	AnnotationImageNamePrefix = AnnotationImageName + "."
+)