This commit aims at improving the user experience:
- Use a dedicated `Dockerfile` for the frontend
- Run the backend and frontend in "watch"/dev mode in Docker
- Do not start all Docker instances for small tasks
README was not correctly updated after last major modif to the admin auth,
resulting in confusion for users trying to log-in with email instead of sub.
Used https://github.com/openfun/joanie as boilerplate, ran a few
transformations with ChapGPT and adapted models and endpoints to
fit to my current vision of the project.