Since Discourse runs in Docker, why not develop there? If you have Docker installed, you should be able to run Discourse directly from your source directory using a Discourse development container.
##### Where is the container image/Dockerfile defined?
The Dockerfile comes from [discourse/discourse_docker on GitHub](https://github.com/discourse/discourse_docker), in particular [image/discourse_dev](https://github.com/discourse/discourse_docker/tree/master/image/discourse_dev).