Author here! I think there's always been a push/pull of "fat base images" versus "install everything every time" - It's obviously subjective, but I think it's

Author here! I think there's always been a push/pull of "fat base images" versus "install everything every time" - It's obviously subjective, but I think it's more important to run the tests on every commit than it is to start the environment from scratch. It's also not necessarily mutually exclusive, you could have a "staging branch" where you make something that looks a lot like production and then re-run end-to-end tests there, while running the per-branch tests with this method to avoid slowing down developers.