🐳 Provide default docker arg

Author: jenssegers

Dockerfile

@@ -1,4 +1,4 @@
-ARG PHP_VERSION
+ARG PHP_VERSION=7.2
 
 FROM php:${PHP_VERSION}-cli
 

docker-compose.yml

@@ -9,8 +9,6 @@ services:
         volumes:
             - .:/code
         working_dir: /code
-        environment:
-          PHP_VERSION: 7.1
         command: bash -c "composer install --prefer-source --no-interaction && php ./vendor/bin/phpunit"
         depends_on:
           - mysql