Developing using Docker

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.

You can find installation instructions and related discussion in this meta topic: https://meta.discourse.org/t/beginners-guide-to-install-discourse-for-development-using-docker/102009

Where is the container image/Dockerfile defined?

The Dockerfile comes from discourse/discourse_docker on GitHub, in particular image/discourse_dev.