Regularly build docker image

[willscott]

we ended up with the docker image no longer building last week without anyone having broken.

We should have a regularly scheduled task in the background that attempts re-building our images so that we can be alerted when anything breaks out from under us.

[kostko]

CircleCI supports scheduling jobs, so we could use it to periodically build images and check that they work? We should not automatically push the built images, just test them.