image: fix image.Manifest compatibility for checkpoint image

Signed-off-by: jerryzhuang <zhuangqhc@gmail.com>

container.go

@@ -330,6 +330,7 @@ func (c *container) Checkpoint(ctx context.Context, ref string, opts ...Checkpoi
 		Versioned: ver.Versioned{
 			SchemaVersion: 2,
 		},
+		MediaType:   ocispec.MediaTypeImageIndex,
 		Annotations: make(map[string]string),
 	}
 	copts := &options.CheckpointOptions{

images/image.go

@@ -207,6 +207,9 @@ func Manifest(ctx context.Context, provider content.Provider, image ocispec.Desc
 
 			var descs []ocispec.Descriptor
 			for _, d := range idx.Manifests {
+				if !IsManifestType(d.MediaType) {
+					continue
+				}
 				if d.Platform == nil || platform.Match(*d.Platform) {
 					descs = append(descs, d)
 				}
@@ -229,7 +232,7 @@ func Manifest(ctx context.Context, provider content.Provider, image ocispec.Desc
 			}
 			return descs, nil
 		}
-		return nil, fmt.Errorf("unexpected media type %v for %v: %w", desc.MediaType, desc.Digest, errdefs.ErrNotFound)
+		return nil, ErrSkipDesc
 	}), image); err != nil {
 		return ocispec.Manifest{}, err
 	}

task.go

@@ -487,6 +487,7 @@ func (t *task) Checkpoint(ctx context.Context, opts ...CheckpointTaskOpts) (Imag
 		Versioned: is.Versioned{
 			SchemaVersion: 2,
 		},
+		MediaType:   v1.MediaTypeImageIndex,
 		Annotations: make(map[string]string),
 	}
 	if err := t.checkpointTask(ctx, &index, request); err != nil {