osbase/oci: implement support for OCI index

Previously, only OCI images were supported, now we can also handle
indexes. The new Ref type is either an Image or Index.

Change-Id: I1b282ed6078d53e9a69e7613f601fdbbe64e192b
Reviewed-on: https://review.monogon.dev/c/monogon/+/4475
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/metropolis/test/localregistry/localregistry.go b/metropolis/test/localregistry/localregistry.go
index 694476e..7440aae 100644
--- a/metropolis/test/localregistry/localregistry.go
+++ b/metropolis/test/localregistry/localregistry.go
@@ -32,7 +32,7 @@
 		if err != nil {
 			return nil, fmt.Errorf("failed to read image from %q: %w", i.Path, err)
 		}
-		err = s.AddImage(i.Repository, i.Tag, image)
+		err = s.AddRef(i.Repository, i.Tag, image)
 		if err != nil {
 			return nil, fmt.Errorf("failed to add image: %w", err)
 		}