allow only china mirrors for docker

defaults/main.yml

@@ -12,3 +12,4 @@ install_docker: false
 upgrade: false
 reboot: false
 pip_executable: pip
+china_mirror_docker: false

tasks/docker.yml

 - name: China mirror for Docker
   set_fact:
     mirror_docker: https://mirrors.tuna.tsinghua.edu.cn/docker-ce
-  when: "china_mirror"
+  when: "china_mirror or china_mirror_docker"
 - name: clean apt Docker
   become: true
   apt: