docs: document debug mode

docs/config.md

@@ -7,3 +7,14 @@ In the Docker image, the `Caddyfile` is located at `/etc/Caddyfile`.
 You can also configure PHP using `php.ini` as usual.
 
 In the Docker image, the `php.ini` file is located at `/usr/local/lib/php.ini`.
+
+## Enable the Debug Mode
+
+When using the Docker image, set the `DEBUG` environment variable to `debug` to enable the debug mode:
+
+```
+docker run -v $PWD:/app/public \
+    -e DEBUG=debug
+    -p 80:80 -p 443:443 \
+    dunglas/frankenphp
+```