Run tests in image built one step above using latest tag

This isn't optimal since theoretically there could be an image pushed
just between building and downloading again, but variables cannot be
used within gitlab-ci.yml so that is what we have to work with. If
variables get introduced, we should lock this to the current commit sha.

.gitlab-ci.yml

@@ -16,10 +16,9 @@ kaniko:
 
 rails spec:
   stage: test
-  image: ruby:2.6.2
+  image: registry.gitlab.com/turniere/turniere-backend/test
   script:
-    - bundle install --path /cache
-    - bundle exec rails db:migrate RAILS_ENV=test
+    - bundle exec rails db:migrate
     - bundle exec rails spec
 
 kubernetes: