Improve code quality of Docker test setup script

[Splines]

This PR improves the code quality of our rspec_inside_docker.py script according to Python11 standards.

Note that we still use the VSCode Test Explorer extension in our setup, while in the meantime, VSCode has its own native Test explorer that Ruby LSP can use. In the long-term, we should migrate to this approach. However:

• Ruby LSP does not support our Docker setup, "only" VSCode dev containers as seen here and we don't migrate to DevContainers soon as discussed in Develop inside a container via Devcontainers #767.
• Ruby LSP also doesn't offer a custom rspec command. While there is a Ruby LSP addon called Ruby LSP RSpec that does support passing an rspecCommand, this doesn't work for us since it uses Ruby LSP internal commands to report back the results. And since Ruby LSP is still running on the host (VSCode) and not inside the Docker Container, this doesn't work.

For these reasons, we stick to the VSCode Test Explorer extension for now, where we return the test results from the docker container as JSON and the extension reads it and reports it in its custom test explorer view.

For more context, see my comment here. I've also created a gist should there be people that want to use our script as well.

Tip

We should monitor Shopify/ruby-lsp#2919 for future developments on this topic, especially see this comment.

[fosterfarrell9]

LGTM