Yes, I agree that it sucks. You might be able to get them to manually allow you to use the prepaid credit card (ref) or unlock your account by contacting billing@fly.io.
Some alternatives:
- Render doesn’t require a credit card (ref) and supports Ruby on Rails, but note that their free PostgreSQL database expires after 90 days (might not be a problem for a demo app); and they require linking with GitHub/GitLab (ref).
- alwaysdata doesn’t require a credit card and supports Ruby on Rails.
- Railway has a free plan. Unverified accounts get a one-off $2 free credits. You can verify by linking to a >=90-day-old GitHub account, then you get $5/month free credits but execution is limited to 500hr/month (= 70% uptime). To lift the execution limit you can buy prepaid credits (min $5; they call this a “credit-based subscription” for the Developer Plan; you’d continue to get $5/month free credits; ref). They support Ruby on Rails (third party video tutorial).