[ci][wip] Rebuild Docker images if necessary

This rebuilds Docker images and uses them in later stages in the same build. If the build is running on `main`, then the images are uploaded to Docker Hub automatically once the run is complete. Images are always rebuilt, but Docker Hub functions as a cache. If there have been no changes to `docker/` since the last available hash on Docker Hub, then the build will just use the images from Hub.

Author: driazati