Use extensions from .devcontainer.json

Often, one will define VS Code extensions that are custom to a project via a `.devcontainer.json` file, as described in

https://code.visualstudio.com/docs/remote/devcontainerjson-reference

I noticed that `github1s` is not using the extensions defined in the `"extensions"` field of `.devcontainer.json` in my projects.

Example:

https://github1s.com/felipepenha/rust-greenhouse/blob/HEAD/.devcontainer.json

Please, notice that this is not a request for Docker integration, but a request for using the extensions required in that specific project.