Added (again) database integration tests

This tries to add tests of the database-module (integration tests) again. They have previously been removed due to issues with the gitlab-runner when waiting for the docker-in-docker (dind) service to start. Further explanation below.

\approve @ds @marius

(approval only if the issue can be fixed on gitlab)

.gitlab-ci.yml

@@ -52,6 +52,7 @@ tests::integration:
   services:
     # We rely on the docker-in-docker service
     - name: docker:20.10.16-dind
+      alias: docker
   before_script:
     # Check if docker is up and running
     - docker version