allow image index type for oci-layout

OCI specification allows either image.index or image.manifest for each manifest descriptor in OCI layout
https://github.com/opencontainers/image-spec/blob/c05acf7eb327dae4704a4efe01253a0e60af6b34/image-layout.md?plain=1#L153

Signed-off-by: Jakub Panek <me@panekj.dev>

Author: panekj

pkg/cmd/builder/build.go

@@ -580,7 +580,7 @@ func parseBuildContextFromOCILayout(name, path string) ([]string, error) {
 
 	var digest string
 	for _, manifest := range ociIndex.Manifests {
-		if images.IsManifestType(manifest.MediaType) {
+		if images.IsManifestType(manifest.MediaType) || images.IsIndexType(manifest.MediaType) {
 			digest = manifest.Digest.String()
 		}
 	}