Drop external Docker builder

[adamnovak]

This beings over some machinery I added in vg to do our Docker builds inside the Gitlab runner containers, using the docker-in-docker setup, instead of doing them on a persistent Kubernetes-hosted builder that needs a lot of resources even when no builds are running. Instead of caching in that persistent builder, it caches to a set of Docker tags on Quay.

After this is merged, we can delete the Kubernetes builder and maybe scale the cluster down some or try moving to Openstack runners.

Changelog Entry

To be copied to the draft changelog by merger:

• Toil CI now builds its Docker images internal to its CI jobs.

Reviewer Checklist

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[DailyDreaming]

LGTM. One minor comment.

[DailyDreaming]

This function doesn't seem to use os anymore?