Automate ./dtox.sh image builds.

[jsirois]

Most of the infra is in place:

• The ./dtox.sh script builds the base image only if needed, it just doesn't push the image.
• The cache image used by CI was built and pushed by hand via tox -e build-cache-image -- --push.

The base image should be built in-line with CI (if needed), but the cache image need not be and is more expensive to build. That can probably be cronned at a period of ~1 week to start.

[jsirois]

d365641 set up a cron job for this but it fails, hitting the GitHub 6 hour time limit.

I'm going to proceed instead with the idea of parallelizing the image build by only building some (or one) tox env per-shard, doing a docker export, then combing tarballs from the docker exports into one big one that is docker imported into 1 image in a fan-in step. A small experiment shows this works to consolidate a cache volume dir from multiple docker builds.