问题：docker ---- It's possible that too few managers are online. Make sure more than half of the managers are online.
背景：docker测试集群，为3台虚拟机，镜像全部拷贝自其它docker 虚拟机。 由于原先安装过swarm，先经过拷贝重启后在执行 dowker swarm XX 时报错。
解决：
1.每台先强制init ：
docker swarm init --force-new-cluster （此时执行 docker node ls --将node信息删除，原因是这些node全部来自我拷贝的镜像中老旧的信息。）
2.然后此时选取 swarm worker节点加入到其中一台时：docker swarm join --token ****
报错Error response from daemon: This node is already part of a swarm. Use "docker swarm leave" to leave this swarm and join another one.
3.根据提示需要 docker swarm leave ，然后报错：
Error response from daemon: You are attempting to leave the swarm on a node that is participating as a manager. Removing the last manager erases all current state of the swarm. Use `--force` to ignore this message.
4.根据提示：加上--force ： docker swarm leave --force 然后再重新join即可
附：Server Version: 17.12.1-ce