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

@@ -38,3 +38,31 @@ tests::unit:
     - pip3 install -r requirements.txt
     # Run the unit tests
     - pytest tests/unit -s -v
+
+tests::integration:
+  stage: test
+  interruptible: true
+  image: docker:20.10.16
+  variables:
+    DOCKER_HOST: tcp://docker:2375
+    DOCKER_DRIVER: overlay2
+    DOCKER_TLS_CERTDIR: ""
+  services:
+    # We rely on the docker-in-docker service
+    - name: docker:20.10.16-dind
+  before_script:
+    # Check if docker is up and running
+    - docker version
+    - docker-compose version
+    # Install dependencies
+    - apk update && apk add py-pip python3-dev libffi-dev
+      gcc libc-dev postgresql postgresql-dev docker-compose postgis
+    # Verify that `psql` executable is there
+    - psql --version && which psql && echo $PATH
+    # Install python pip
+    - python3 -m pip install --upgrade pip
+  script:
+    # Install Python `pytest` docker, mocker and database libraries
+    - pip3 install -r requirements.test-integration.txt
+    # Run the integration test
+    - pytest tests/integration -s -v