Switch to my own images, with dependencies satisfied

Necessary PHP modules, as well as Composer, are provided in the new images: https://git.ethitter.com/docker/images/container_registry.

.gitlab-ci.yml

@@ -6,6 +6,7 @@ variables:
 cache:
   paths:
     - $HOME/.composer
+    - /root/.composer
 
 before_script:
   # Set up WordPress tests