Add format checking to CI.

.gitlab-ci.yml

@@ -9,6 +9,7 @@ variables:
   POSTGRES_PASSWORD: postgres
 
 stages:
+  - lint
   - test
 
 before_script:
@@ -18,6 +19,11 @@ before_script:
   - MIX_ENV=test mix ecto.create
   - MIX_ENV=test mix ecto.migrate
 
+lint:
+  stage: lint
+  script:
+    - MIX_ENV=test mix format --check-formatted
+
 unit-testing:
   stage: test
   script: