From https://devcenter.heroku.com/articles/scheduler
Scheduler is a best-effort service. There is no guarantee that jobs will execute at their scheduled time, or at all.
heroku-cron is a simple kind-of custom-clock process, which one can use to run shell commands (which could schedule jobs, or do work themselves) a la cron.
You'll need docker to heroku compatible binary. To perform build just ./build.sh.