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/cloud/agent/install.go b/cloud/agent/install.go
index 9ada49b..9144769 100644
--- a/cloud/agent/install.go
+++ b/cloud/agent/install.go
@@ -21,6 +21,7 @@
 	metropolisInstall "source.monogon.dev/metropolis/installer/install"
 	"source.monogon.dev/osbase/blockdev"
 	"source.monogon.dev/osbase/efivarfs"
+	"source.monogon.dev/osbase/oci"
 	"source.monogon.dev/osbase/oci/osimage"
 	"source.monogon.dev/osbase/oci/registry"
 	"source.monogon.dev/osbase/structfs"
@@ -75,7 +76,7 @@
 		Repository: req.OsImage.Repository,
 	}
 
-	image, err := client.Read(ctx, req.OsImage.Tag, req.OsImage.Digest)
+	image, err := oci.AsImage(client.Read(ctx, req.OsImage.Tag, req.OsImage.Digest))
 	if err != nil {
 		return fmt.Errorf("failed to fetch OS image: %w", err)
 	}