Add privileged mode for Docker-in-Docker support

Certain tools, like code quality, require dind support.

https://gitlab.com/gitlab-org/gitlab-runner/issues/1261

tasks/register-runner.yml

@@ -15,6 +15,7 @@
     --executor '{{ gitlab_runner_executor }}'
     --limit '{{ gitlab_runner_concurrent_specific }}'
     --locked='{{ item.locked | default( gitlab_runner_locked ) }}'
+    --docker-privileged
     --docker-image '{{ gitlab_runner_docker_image }}'
     {% if gitlab_runner_docker_volumes is defined %}
     --docker-volumes "{{ gitlab_runner_docker_volumes | join('" "') }}"