docker: allow for privileged docker engine

For docker-in-docker, privileged mode must be enabled.[0]

This commit allows for privileged mode to be enabled for the docker
executor, via an ansible.fact.

[0] https://docs.gitlab.com/runner/executors/docker.html#the-privileged-mode

roles/gitlab-runner/templates/gitlab-runner-docker.toml

@@ -13,6 +13,6 @@ log_level = "info"
 
   [runners.docker]
     image = "debian:stable"
-    privileged = false
+    privileged = {{ ansible_local.gitlab_runner.privileged | default('false') }}
     disable_cache = false
     shm_size = 0