=== Docker alert ===
If you're using docker,
I think it is proper to add that I couldn't manage to make cron to run in the background.
To run a cron job inside the container, I used supervisor and ran cron -f, together with the other process.
Edit: Another issue - I also didn't manage to get it work when running the container with HOST networking. See this issue here also: https://github.com/phusion/baseimage-docker/issues/144