Add Xdebug instruction and proper Docker Compose configuration

README.md

@@ -55,3 +55,6 @@ services:
 ## Updating the submodule
 This site uses the master branch of the [main PHP-FIG repository](https://github.com/php-fig/fig-standards) as a Git submodule to pull content from it. 
 If you need to update it, you can use the `bin/update_submodule.sh` script to pull new commits. Afterwards, you can commit the differences in this repo, to push the updates afterwards.
+
+## Using Xdebug
+If you need to debug the site build, you can enable Xdebug. To do it, you need to copy `docker-compose.override.dist.yml` as `docker-compose.override.yml` so that your local Docker Compose configuration loads that configuration. This (after a container reload) will put Xdebug in `debug` mode. Once done that, you need to configure your IDE to catch to that. Xdebug will automatically connect back your host system thanks to the `host.docker.internal` special hostname, with a serverName of `PHP-FIG`, as defined in `docker-compose.yml`.

docker-compose.override.dist.yml

@@ -0,0 +1,6 @@
+version: '3.5'
+
+services:
+  php:
+    environment:
+      XDEBUG_MODE: 'debug'

docker-compose.yml

@@ -7,6 +7,11 @@ services:
     user: fig
     volumes:
     - .:/fig-website
+    - ./docker/php/xdebug.ini:/usr/local/etc/php/conf.d/xdebug.ini
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
+    environment:
+      PHP_IDE_CONFIG: "serverName=PHP-FIG"
     working_dir: /fig-website
     command:
       - 'sculpin'