* Use `production` docker images for cypress tests. As one of our contributors
pointed out: It is bad practice to end-to-end test sth. else than the actual
production images.
* Have a dedicated `docker-compose.travis-integration.yml` to test
backend integration tests. The backend *needs* a database when doing
integration tests. So there is no way around using docker-compose. But
we could stop using the `build-and-test` stage when we do cypress tests.
* Remove a couple of unused ports and DRY `docker-compose.override.yml`
* Build and tag all images first and then run `docker-compose .. up` - I
hope this will not rebuild the image.
* Reduce docker image size
