Integration testing of multiple containers via docker in docker

About a year ago we set up a gitlab-pipeline to integration-test a bunch of interacting containers. The whole test is steered from a pytest based script which starts/stops individual containers and feeds them with data.

Now we need to move this whole integration test into a docker container to ensure isolation from different pipelines. For this we planned to use gitlabs docker in docker workflow.

However, with this workflow the gitlab-runner now spawns a separate container for the pytest engine as well as a container which then holds all spawned sub-containers. This leads to the problem that the steering script container is unable to connect to the testie containers inside the service container.

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Maybe I misunderstood parts of the documentation and everything is just very easy, so please bare with me. How can I add this connection and allow the build-container to connect to one of the testie containers?

The relevant part of the .yaml is this:

  stage: integration_test
  image: <image path>dind-py3-cml
    - docker:dind
    - pytest --capture=sys ./unittests/integration_test/