tests: cache Docker images

[slint]

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).