Add comment about timezone definition

docker-compose.yml

@@ -15,6 +15,7 @@ services:
         build:
             context: docker/php
             args:
+                # Set here your timezone using one of this: http://php.net/manual/en/timezones.php
                 timezone: 'Europe/Monaco'
         ports:
             - "9000:9000"

docker/php/Dockerfile

@@ -1,5 +1,6 @@
 FROM php:fpm
 
+# Default timezone. To change it, use the argument in the docker-compose.yml file
 ARG timezone='Europe/Paris'
 
 RUN apt-get update && apt-get install -y \