refactor: add default value for parent-image

Author: zynaa

.github/workflows/docker-build-test-upload.yml

@@ -13,8 +13,9 @@ on:
         type: string
       parent-variant:
         description: Parent variant tag prefix
-        required: true
+        required: false
         type: string
+        default: default
       image:
         description: Image name
         required: true