GitLab: update host for MySQL container

See https://hub.docker.com/r/_/mysql/

.gitlab-ci.yml

@@ -15,7 +15,7 @@ before_script:
 # Install Composer
 - curl -sS https://getcomposer.org/installer | php
 # Prepare to run WP tests
-- bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
+- bash bin/install-wp-tests.sh wordpress_test root '' $MYSQL_PORT_3306_TCP_ADDR $WP_VERSION
 
 # Bring in any services we need http://docs.gitlab.com/ee/ci/docker/using_docker_images.html#what-is-a-service
 # See http://docs.gitlab.com/ce/ci/services/README.html for examples.