Stop scheduling mirror tests while building Docker images

The mirror tests start by build Docker images. Scheduling it at midnight
will make it run at the same time as the job building the main Docker
images. This mean they will fight for access to the Docker daemon and
thus one of them is likely to fail.

Ideally, we would use the lockable resource plugin but as a stopgap
measure, let’s schedule the mirror tests 4 hours after the main image
building job.