GitLab CI: Provide sample config

If it is missing, even the assets:precompile will fail.

.gitlab-ci.yml

@@ -10,6 +10,7 @@ before_script:
   - gem install bundler
   - bundle -v
   - bundle install -j $(nproc) --path vendor
+  - cp config.sample.yml config.yml
 
 rubocop:
   script: