Fixes value for storage.oci.repository

Previously while providing repo url value for storage
oci repository, chains controller was giving an error
as

`a digest must contain exactly one '@' separator (e.g.
registry/repository@digest)`

because, it was not able to add digest and repo name

Hence this patch fixes it, by formatting the value
provided by the user and thus storing the
attestations/signatures in the provide location

Signed-off-by: PuneetPunamiya <[email protected]>

pkg/chains/storage/oci/legacy.go

@@ -18,6 +18,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"strings"
 
 	"github.com/tektoncd/chains/pkg/chains/formats"
 	"github.com/tektoncd/chains/pkg/chains/objects"
@@ -280,8 +281,11 @@ func (b *Backend) RetrieveArtifact(ctx context.Context, obj objects.TektonObject
 
 func newDigest(cfg config.Config, imageName string) (name.Digest, error) {
 	// Override image name from config if set.
-	if r := cfg.Storage.OCI.Repository; r != "" {
-		imageName = r
+	if storageOCIRepository := cfg.Storage.OCI.Repository; storageOCIRepository != "" {
+		parts := strings.SplitN(imageName, "/", 3)
+		if len(parts) == 3 && (strings.ContainsRune(parts[0], '.') || strings.ContainsRune(parts[0], ':')) {
+			imageName = fmt.Sprintf("%s/%s", cfg.Storage.OCI.Repository, parts[2])
+		}
 	}
 
 	var opts []name.Option