message: Use --offline when building docs
  This reflects the intent of the command, since containers are isolated from the
  network. And it provides a clearer error message in case there is some problem
  and cargo would try to make a network request.
diff: @@ -673,6 +673,7 @@ impl RustwideBuilder {
  // Add docs.rs specific arguments
  let mut cargo_args = vec![
  + "--offline".into(),
  // We know that `metadata` unconditionally passes `-Z rustdoc-map`.
  // Don't copy paste this, since that fact is not stable and may change in the future.
  "-Zunstable-options".into(),
  