How does the free tier work? Can you sign up so that you only use the free tier, and builds beyond the limit are queued for the next day or don't occur at all?

How does the free tier work? Can you sign up so that you only use the free tier, and builds beyond the limit are queued for the next day or don't occur at all? Or do you need to have a "card on file" to run the free tier at all, with the chance that you'll have to pay if you somehow exceed the free tier (stuck build, etc)?