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.