Add a button to trigger a build with an arbitrary environment variable.
For example, I could add a "Rebuild without Docker Layer Cache" button, check for NO_DOCKER_LAYER_CACHE in my circle.yml file, and conditionally remove old docker images.
Currently there does not seem to be a way to clear the docker layer cache without multiple commits to circle.yml (add clearing line, remove clearing line). This is insane and will drive us off of CircleCI if not addressed. A parameterized build button solves this and many other problem scenarios.