probably that is the way to do it

.gitlab-ci.yml

@@ -10,11 +10,13 @@ before_script:
 # Install git, the php image doesn't have installed
 - apt-get update -yqq
 - apt-get install git -yqq
-- apt-get install php7.0-gd -yqq
 
 # Install mysql driver
 - docker-php-ext-install pdo_mysql
 
+# Install php-gd
+- docker-php-ext-install gd
+
 # Install composer
 - curl -sS https://getcomposer.org/installer | php