Nothing stops you from using cron in your Dockerfile:
RUN apt-get install -y cron
Actually I just did it a few weeks ago and it works as expected.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.
Don't need to 'rail against' someone building a product that doesn't exactly fit your needs.
So, here is what I did. Added a simple API layer in front of my script. The API layer takes in a secret message to a specific endpoint that initiates a background process. Initially, I used my raspberry pi to periodically make the API requests. But I plan to try out: https://cron-job.org/en/
Having heard about fly.io from fasterthanlime I was expecting things to be smoother but there were a few gotchas.
Eventually I managed to configure things in the right way but it was a far cry from heroku buildpacks.
I'm trying to get my clients (1M+ spending per year) to switch from azure (absolute dumpster fire) to fly.io but I'm not sure it's mature enough just yet.
At that point I'd rather have to configure standard machines on hetzner than finding the quirks of a random paas
The best aspects of fly.io are 1) effortless multi-regional deployments 2) technological freedom 3) price. For example: Azure = $140, Fly.io = $10 for the same deployment. We are in love with those guys and gals.
But it's not all sunshine and roses. Some fly.io users have intermittent problems during deployments (see forum). I believe those issues will be rectified and improved with time.