Automatically keeping a GitHub fork up to date

We recently setup a departmental GitHub account for Hutton ICS, and one of the things we'll use this for is to showcase projects which ICS staff are contributing to - such as Biopython in my case.

To start with we have forked https://github.com/biopython/biopython as https://github.com/huttonics/biopython which we'll use as a read-only mirror - but now we want to keep it up to date with commits pushed to the upstream repository.

How can we automatically mirror the upstream repository? Enter GitHub Deploy Keys, which we can use to grant read/write access on a repository basis - which a cron job can use to push changes to our mirrored git repository.