Redo the run-tests workflow

Currently, the workflow checks out a very heavy front-end action that builds an uncached docker image every time it runs, taking more than 3 mins -> https://github.com/GIScience/github-action-webapp-front-end-test/

The best would be to implement the Dockerfile from issue #291 first, and let the testing environment run in the same docker image that will be used for production.

Best is to cache the individual layers to only rebuild if needed, and could also be used to push images to docker hub if we'd like to.