c8d/builder: Don't drop fields from created image

Previous image created a new partially filled image.
This caused child images to lose their parent's layers.

Instead of creating a new object and trying to replace its fields, just
clone the original passed image and change its ID to the manifest
digest.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

daemon/containerd/image_builder.go

@@ -493,9 +493,6 @@ func (i *ImageService) CreateImage(ctx context.Context, config []byte, parent st
 		return nil, err
 	}
 
-	newImage := dimage.NewImage(dimage.ID(createdImage.Target.Digest))
-	newImage.V1Image = imgToCreate.V1Image
-	newImage.V1Image.ID = string(createdImage.Target.Digest)
-	newImage.History = imgToCreate.History
+	newImage := dimage.Clone(imgToCreate, dimage.ID(createdImage.Target.Digest))
 	return newImage, nil
 }

image/image.go

@@ -259,6 +259,15 @@ func NewChildImage(img *Image, child ChildConfig, os string) *Image {
 	}
 }
 
+// Clone clones an image and changes ID.
+func Clone(base *Image, id ID) *Image {
+	img := *base
+	img.RootFS = img.RootFS.Clone()
+	img.V1Image.ID = id.String()
+	img.computedID = id
+	return &img
+}
+
 // History stores build commands that were used to create an image
 type History = ocispec.History