Add shared content label to namespaces

Adds shared content labels to namespaces allowing content to be shared
between namespaces if that namespace is specifically tagged as being
sharable by adding the `containerd.io/namespace/sharable` label to the
namespace.

Signed-off-by: Cody Roseborough <cdr@amazon.com>

metadata/images.go

@@ -55,6 +55,28 @@ func (s *imageStore) Get(ctx context.Context, name string) (images.Image, error)
 
 	if err := view(ctx, s.db, func(tx *bolt.Tx) error {
 		bkt := getImagesBucket(tx, namespace)
+		if bkt == nil || bkt.Bucket([]byte(name)) == nil {
+			nsbkt := getNamespacesBucket(tx)
+			cur := nsbkt.Cursor()
+			for k, _ := cur.First(); k != nil; k, _ = cur.Next() {
+				// If this namespace has the sharedlabel
+				if hasSharedLabel(tx, string(k)) {
+					// and has the image we are looking for
+					bkt = getImagesBucket(tx, string(k))
+					if bkt == nil {
+						continue
+					}
+
+					ibkt := bkt.Bucket([]byte(name))
+					if ibkt == nil {
+						continue
+					}
+					// we are done
+					break
+				}
+
+			}
+		}
 		if bkt == nil {
 			return errors.Wrapf(errdefs.ErrNotFound, "image %q", name)
 		}