metadata: validation and testing of image store

Signed-off-by: Stephen J Day <stephen.day@docker.com>

images/image.go

@@ -15,9 +15,25 @@ import (
 
 // Image provides the model for how containerd views container images.
 type Image struct {
-	Name                 string
-	Labels               map[string]string
-	Target               ocispec.Descriptor
+	// Name of the image.
+	//
+	// To be pulled, it must be a reference compatible with resolvers.
+	//
+	// This field is required.
+	Name string
+
+	// Labels provide runtime decoration for the image record.
+	//
+	// There is no default behavior for how these labels are propagated. They
+	// only decorate the static metadata object.
+	//
+	// This field is optional.
+	Labels map[string]string
+
+	// Target describes the root content for this image. Typically, this is
+	// a manifest, index or manifest list.
+	Target ocispec.Descriptor
+
 	CreatedAt, UpdatedAt time.Time
 }