For me, the key was discovering that while both my native and docker builds fail, my Dockerfile.render builds (which use the heroku buildpack) magically work AND I could use that Dockerfile.render build for the Cron Job (I don't think I would have figured out that I could just plop in that Dockerfile Path in the Advanced section).
I have no experience with Docker (someone set up docker-compose stuff for us like 4 years ago as a student project, but I haven't used it). So it was a little overwhelming for me when the Render migration tool thrust Docker on me.
Still testing out some things before configuring my DNS. I'm sure in a week you'll never hear from me again (because things will just work). I enjoy how snappy the Render interface is. Maybe more of a walkthrough (with screenshots) on the cron jobs doc?
