Yes, CircleCI creates a Docker container for every run. In the CircleCI 2.0 config style it is more explicit about this behavior (one specifies the Docker image they want to use) while in the 1.0 API, the config was more inferred / implicit. It can get more complicated than this, but at minimum every build on CircleCI is creating and destroying at least one container to run the unit test suite. It can do this pretty quickly though: a barebones Python repo (1 function, 22 parameterized unit tests) for me takes ~12 seconds from startup to teardown to displaying the completed build using 2.0 with virtual environment caching.
Sorry, I meant to ask about locally. I am still trying to figure out how to write code in docker. It is very confusing and send like too much work to do this locally.
I assume the disk is pretty much always the bottleneck? I mean even with system CTL, how much memory can a rest API flask web app take right?
I'm not sure how it worked prior to 2.0 or if it was possibly, but you can build locally in 2.0 ($ circleci build) which I think is just a thin wrapper around a docker build + docker run + run unit tests (or whatever you want).
That said, you don't have to use Docker and for a small project with no dependencies or no system level dependencies, I think it's overkill personally, so I'd just test outside of Docker or use the new base images from Circle.
