Investigate using a custom image for the zephyr job

[wmamills]

Currently the zephyr clone takes 7 or 8 minutes.

We could look at creating a docker image with a pre-cloned zephyr setup. We could still update in the job but updating is much faster than cloning from scratch. There should be a job that runs one a month or so that updates the zephyr docker image.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.