Add clarification comment

.github/workflows/build-subworkflow.yml

@@ -9,6 +9,9 @@ on:
       docker-image:
         required: true
         type: string
+      # User id is used to run the docker container as the self-hosted user
+      # It allows to delete build files created in the docker while on the host (checkout-repository job)
+      # By default, those files are owned by the docker user, and root privileges are required to delete them
       self-hosted-user-id:
         required: true
         type: string