Dockerize your unit tests for faster feedback

I think this was a really useful talk, and well-delivered. I liked how you described the process by which you came up with your current solution, and the iterative improvements you made. Nevertheless, there are some things that could be improved in my humble opinion:

1) I think it would be good to make a distinction between the speedup (due to parallellisation) and other advantages provided by Docker (isolation, testing on different PHP versions, etc.). Running test groups in parallel is probably also possible without Docker (using GNU 'parallel' alone), so it would be good to 'justify' the use of Docker on other grounds.

2) If I read your slides correctly, it seems that the first complete test run (plain PHPUnit, all tests) is actually faster than the last example (Docker, in parallel, with output shown). That feels strange, given the goal of making tests run faster. Maybe the trick you mentioned to speed up Docker container startup time can improve on this.

Thank you for the useful content!