Tox handles setting up a virtual environment for python dependencies, installing dependencies, building the docker images used by tests, and finally running tests. It's an easy way to have travis-ci like build behavior locally.
The build_stage tests have to run first to create the docker images, followed by the actual tests which utilize said images. Unless you're changing your dockerfiles you shouldn't have to run the build_stage every time - but it's a good idea to rebuild at least once a day in case the base Docker images or packages change.