Improve Docker build times in CI

[elldritch]

Docker builds are really slow right now because they start from scratch. Caching seems like the obviously correct approach here (and the Dockerfile is structured to be conducive to caching, which works well locally), but I can't seem to get it to work.

Some references I found while investigating:

• https://github.com/dtinth/github-actions-docker-layer-caching-poc/blob/master/.github/workflows/dockerimage.yml
• https://dev.to/dtinth/caching-docker-builds-in-github-actions-which-approach-is-the-fastest-a-research-18ei
• docker/build-push-action#7
• docker/build-push-action#50

[elldritch]

Fixed in 9aa44f5.