CI: test with images

.gitlab-ci.yml

-image: python:latest
-
 stages:
   - test
 
@@ -27,6 +25,7 @@ cache:
 
 test:
   stage: test
+  image: mdillon/postgis
   before_script:
     # Install dependencies and create associated cache dirs
     - mkdir -p "${APT_STATE_LISTS}/partial"