Add docker build cache to github actions cache to improve CI build times

[byron-marohn]

Difficulty: Medium to Hard depends are understanding of GitHub Actions.

Potential enhancement idea for future work - it seems like it should be possible to cache part of the docker build layers to improve build times using the GitHub Actions cache system. The docker build is far slower than all the other build steps combined.

Tricky part is to do so in a way that doesn't eventually fill up the actions cache. We were thinking about trying to do this but ran out of time. Here's a decent article with a possible approach to this - https://evilmartians.com/chronicles/build-images-on-github-actions-with-docker-layer-caching

Maintainers, feel free to close or comment on this if you would be interested in this in the future.