Function next_message 

async fn next_message(
    attach: &SharedContainerAttach,
    rx: &mut Receiver<Message>,
    drain: Option<&mut Drain>,
    lag: &mut Lag,
) -> Result<Message, RecvError>

The next output message, bounded by the drain once the container exited. If the output did not end after the exit, the client gets the done packet anyway.