Remove the `docker` dev dependency

[escattone]

The docker Python package is part of our dev dependencies, but it's not used by any code within the kitsune repo, including not by any of the other installed Python packages.

It was added on April 2023 for Selenium IP discovery, but the consuming code was removed during the Selenoid migration in September of 2023, and now the whole Selenium suite is gone — it's been an unused leftover ever since.