testing: one docker for testing is (more than) enough

Testing would be greatly ! improved if we don't build all those dockers for inter-compatibility testing, but instead create only one with everything what's needed in it.

No files, tests, or entry points are named. Inspect the repository's Docker-based interoperability testing setup to identify how the current images are built and used; done means the testing workflow uses one Docker image containing all required dependencies and still passes.

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