This codespace demo uses a hosted image, instead of creating the docker container from scratch. A side effect of this is, that you can not use it locally with "Remote Containers".
How To Create The Codespace
If you have codespaces enabled on github, you should see the "Create codespace on master" button (see pic). If you don't have codespaces, you can apply for a beta membership as an individual or use the "Run locally" method.
Beware, the preparation of the Codespace can take a few minutes. Also check the github pricing, as Codespaces may encur costs (post-beta).
Debug The Project
Once the container has been built, you will be in a Visual Studio Code environment, running in your browser. You can now build and debug the project: