Add support for caching with Github Actions cache

[orf]

This MR adds support for caching with Github Actions, rather than pulling/pushing the image to a registry.

GHA cache is quicker and is integrated with Docker buildx.

The downside is that we need to use the docker setup buildx action, because adding this to the shell script seems somewhat complex?

You can see the caches on my fork.

[covers1624]

I really like this idea, it would speed up my own builds quite a lot.

FWIW, You can switch the action to using composite steps, and pre-install docker-buildx, instead of requiring that its installed externally. There are some docs on doing this over here: https://docs.github.com/en/actions/sharing-automations/creating-actions/metadata-syntax-for-github-actions#runsstepsuses