## 1. Frontend Application Changes

- [x] 1.1 In `frontend/src/api.js`, modify the logic to use `process.env.REACT_APP_BACKEND_URL` for the API base URL.
- [x] 1.2 Implement a fallback to `http://localhost:3001` in the same file for when the environment variable is not set.

## 2. Docker Configuration

- [x] 2.1 In `docker-compose.yml`, update the `frontend` service definition to pass the `REACT_APP_BACKEND_URL` environment variable.
- [x] 2.2 Add comments to `docker-compose.yml` to clarify the purpose and usage of the new environment variable.
- [x] 2.3 In `frontend/Dockerfile`, add `ARG` and `ENV` to pass the `REACT_APP_BACKEND_URL` to the build process.

## 3. Documentation

- [x] 3.1 Update the main `README.md` to document the new `REACT_APP_BACKEND_URL` environment variable, explaining its purpose and how to use it.