Add debugging section to the core manual

resolves #551

Author: hschaeidt

core/debugging.md

@@ -0,0 +1,64 @@
+# 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 API Platform to production with an active Xdebug extension,
+it's recommended to add a custom stage to the end of the `api/Dockerfile`.
+
+```Dockerfile
+# api/Dockerfile
+FROM api_platform_php as api_platform_php_dev
+
+ARG XDEBUG_VERSION=2.6.0
+RUN apk add --no-cache $PHPIZE_DEPS; \
+    pecl install xdebug-$XDEBUG_VERSION; \
+    docker-php-ext-enable xdebug
+```
+
+## Configure Xdebug with docker-compose override
+
+Using a [override](https://docs.docker.com/compose/reference/overview/#specifying-multiple-compose-files)  
+ file named `docker-compose.override.yml` ensures that the production
+configuration remains untouched.
+
+As example, an override could look like this:
+
+```yml
+version: '3.4'
+ services:
+  php:
+    build:
+      target: api_platform_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
+      # The `remote_host` below may optionally be replaced with `remote_connect_back`
+      #
+      # Choose some other port than `9000`, as this one is already occupied by FPM
+      # Make sure to also change your listening port in the IDE to this value
+      XDEBUG_CONFIG: remote_enable=1
+        remote_host=host.docker.internal
+        remote_port=9999
+        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=api-platform
+```
+
+## 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
+```