add env var

README.md

@@ -39,6 +39,22 @@ To run this application, you need to have the following installed:
     -   **Frontend**: Open your web browser and navigate to `http://localhost:3000`.
     -   **Backend API**: The backend API is available at `http://localhost:3001/api/services`.
 
+## Configuration
+
+The application can be configured using environment variables.
+
+### Frontend
+
+-   `REACT_APP_BACKEND_URL`: The URL of the backend API. This is passed to the React application during the build process. If not set, the frontend will default to `http://localhost:3001`.
+
+    To set this variable when running with Docker Compose, you can create a `.env` file in the root of the project with the following content:
+
+    ```
+    REACT_APP_BACKEND_URL=http://your-backend-url:3001
+    ```
+
+    Then run `docker compose up --build`.
+
 ## Accessing Services
 
 -   The frontend will display cards for each discovered Docker service.