[Docker] Use default Rails Devcontainer setup

Rails 7.2 (https://github.com/hackclub/hcb/issues/7691) comes with a default devcontainers config

https://github.com/rails/devcontainer
https://edgeguides.rubyonrails.org/getting_started_with_devcontainer.html

---

Let's still keep the root `Dockerfile`. The goal is to replace our weird patchy devcontainer config with a (probably) more robust one