tests: cache Docker images

## Is your feature request related to a problem? Please describe.

In the `tests` GH actions there are many times where the underlying tests use Docker images for their services (PostgreSQL, ElasticSearch, etc.). Since these don't change that often because we're controlling their versions via `docker-services-cli`, it would be nice to cache them also.

## Describe the solution you'd like

Using something like https://github.com/marketplace/actions/docker-layer-caching is probably the quickest/safest way to get this in.

## Describe alternatives you've considered

There's no "official" GitHub configuration in the `actions/cache` repo (which the repo mentioned above actually uses).