Nothing stops you from using cron in your Dockerfile:
RUN apt-get install -y cron
fly.io is universal, it just runs your Dockerfile. Everything else is up to you.
It's weird, because there's a group of (understandably) devoted Heroku users who have very high standards for fidelity to Heroku's DX, and while we're thrilled to do our best to meet (or at least approach) those standards, being Heroku is actually not the primary mission of our team; getting people's apps running quickly and close to their users globally is.
It only supports hourly, daily, or every 10 minutes and it isn't even guaranteed to run. I would see it skip jobs more often than I expected, maybe 1 in 20 at times.
Supporting actual cron is way better than anything Heroku offered.