docker: set volumes

set "/cache" as a volume by default, and allow for override via ansible
fact

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

@@ -15,4 +15,5 @@ log_level = "info"
     image = "{{ ansible_local.gitlab_runner.image | default('debian:stable') }}"
     privileged = {{ ansible_local.gitlab_runner.privileged | default('false') }}
     disable_cache = false
+    volumes = [{{ ansible_local.gitlab_runner.volumes | default('"/cache"') }}]
     shm_size = 0