Add cron job back with supercronic

[ian-cawood]

Hi there this dockerfile really saved me. I like the simplicity and the functionality is pretty much exactly what I needed. It works great for multiple server blocks as well.

I recently found and used https://github.com/aptible/supercronic to do cron inside a docker container. It's build specifically for running cron jobs in containers. I needed mine for a php schedule. But I think it can solve your case as well.