Make Dockerfile compatible with PHPUnit 9

Author: stephandesouza

Diff for: Dockerfile

@@ -1,5 +1,5 @@
-ARG PHP_VERSION=7.2
-ARG COMPOSER_VERSION=1.8
+ARG PHP_VERSION=7.3
+ARG COMPOSER_VERSION=2.0
 
 FROM composer:${COMPOSER_VERSION}
 FROM php:${PHP_VERSION}-cli