Add documentation for debugging

Author: maxhelias

README.md

@@ -23,6 +23,79 @@ For instance, use the following command to use the `master` branch of Symfony:
 
 `STABILITY=dev docker-compose up --build`
 
+## Debugging
+
+The default Docker stack is shipped without a Xdebug stage. It's easy
+though to add [Xdebug](https://xdebug.org/) to your project, for development
+purposes such as debugging tests or API requests remotely.
+
+### Add a Development Stage to the Dockerfile
+
+To avoid deploying Symfony Docker to production with an active Xdebug extension,
+it's recommended to add a custom stage to the end of the `Dockerfile`.
+
+```Dockerfile
+# Dockerfile
+FROM symfony_docker_php as symfony_docker_php_dev
+
+ARG XDEBUG_VERSION=2.6.0
+RUN set -eux; \
+	apk add --no-cache --virtual .build-deps $PHPIZE_DEPS; \
+	pecl install xdebug-$XDEBUG_VERSION; \
+	docker-php-ext-enable xdebug; \
+	apk del .build-deps
+```
+
+### Configure Xdebug with Docker Compose Override
+
+Using an [override](https://docs.docker.com/compose/reference/overview/#specifying-multiple-compose-files)  
+ file named `docker-compose.override.yaml` ensures that the production
+configuration remains untouched.
+
+As example, an override could look like this:
+
+```yaml
+version: '3.4'
+
+services:
+  php:
+    build:
+      context: .
+      target: symfony_docker_php_dev
+    environment:
+      # See https://docs.docker.com/docker-for-mac/networking/#i-want-to-connect-from-a-container-to-a-service-on-the-host
+      # See https://github.com/docker/for-linux/issues/264
+      # The `remote_host` below may optionally be replaced with `remote_connect_back`
+      XDEBUG_CONFIG: >-
+        remote_enable=1
+        remote_host=host.docker.internal
+        remote_port=9001
+        idekey=PHPSTORM
+      # This should correspond to the server declared in PHPStorm `Preferences | Languages & Frameworks | PHP | Servers`
+      # Then PHPStorm will use the corresponding path mappings
+      PHP_IDE_CONFIG: serverName=symfony-docker
+```
+
+And run :
+````bash
+docker-compose -f docker-compose.yaml -f docker-compose.override.yaml up -d
+````
+
+### Troubleshooting
+
+Inspect the installation with the following command. The requested Xdebug
+version should be displayed in the output.
+
+```bash
+$ docker-compose exec php php --version
+
+PHP 7.2.8 (cli) (built: Jul 21 2018 08:09:37) ( NTS )
+Copyright (c) 1997-2018 The PHP Group
+Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
+    with Zend OPcache v7.2.8, Copyright (c) 1999-2018, by Zend Technologies
+    with Xdebug v2.6.0, Copyright (c) 2002-2018, by Derick Rethans
+```
+
 ## Credits
 
 Created by [Kévin Dunglas](https://dunglas.fr) and sponsored by [Les-Tilleuls.coop](https://les-tilleuls.coop).