fix: mark image-ref attribute optional (#261)

Author: arxeiss

Diff for: action.yaml

@@ -8,7 +8,7 @@ inputs:
     default: 'image'
   image-ref:
     description: 'image reference(for backward compatibility)'
-    required: true
+    required: false
   input:
     description: 'reference of tar file to scan'
     required: false