Choose random host port in tests

[mathbunnyru]

Related: #1622

[consideRatio]

LGTM!

Note for other reviewers and as a verification of my understanding by making it explicit: when we pass ports={"8888/tcp": None} to run_detached, it means that the container port 8888/tcp should be associated with the port None (random) in the host port.

docker-stacks/conftest.py

Lines 86 to 89 in b020a0c

	self.container = self.docker_client.containers.run(
	self.image_name,
	**all_kwargs,
	)

    running_container = container.run_detached(
        command=["start-notebook.sh", "--NotebookApp.token=''"],
        ports={"8888/tcp": None},
    )

[mathbunnyru]

Thank you @consideRatio!

I want to confirm that after this change I am able to run make test/base-notebook in several tabs without any errors, which almost certainly means that we can run tests in parallel, both for the samge image or for different images as well.

[mathbunnyru]

Tests pass, so merging this :)