use spec's ref instead of canonical ref when copying catalog image

When used in conjuction with a "tag-only" registries.conf mirror
registry configuration, our containers/image unpacker fails. This is
because we resolve a canonical ref from the tag, and then use the
digest-based reference to actually copy the image, thus thwarting the
tag-only configuration.

This commit ensures that we always copy the image using the reference
provided in the spec.

Signed-off-by: Joe Lanford <joe.lanford@gmail.com>

internal/source/containers_image.go

@@ -88,7 +88,7 @@ func (i *ContainersImageRegistry) Unpack(ctx context.Context, catalog *catalogdv
 	// copy.Image can concurrently pull all the layers.
 	//
 	//////////////////////////////////////////////////////
-	dockerRef, err := docker.NewReference(canonicalRef)
+	dockerRef, err := docker.NewReference(imgRef)
 	if err != nil {
 		return nil, fmt.Errorf("error creating source reference: %w", err)
 	}