Rebuild gcloud-docker less frequently to avoid day-0 bugs

We currently rebuild all images nightly, which means that if there is a day-0 bug all services that make use of an image will receive that bug immediately.

For gcloud-docker we don't need to be on the bleeding edge, and rebuilding weekly could mean that we avoid bugs that will be fixed in follow up releases.

This is being raised in response to a bug in gcloud causing problems in the sync jobs for deploy-identity: https://gitlab.developers.cam.ac.uk/uis/devops/iam/deploy-identity/-/issues/28