Add docker container to tests (and run them from within!)

[phinate]

Currently, we copy a lot of CI from poke-env to setup the showdown side of things. I think it would be much better to incorporate the necessary aspects of this into our Dockerfile written by @boykovdn, and then run the tests in that docker container, building it on the fly for every test run.

This way, we keep our Docker image shiny and updated, while also making a possibly re-usable pokemon showdown github action! See e.g. https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action

[phinate]

@hsahovic, let me know if that's something you'd be interested in! And thanks for the nicely written CI, made it very easy for us to set up testing :)

[hsahovic]

Thanks! If you set something like this up, let me know - I might want to implement something similar in poke-env :)