service docker start 报错 ：Failed to start docker.service: Unit not found.

解决办法：

     vi /usr/lib/systemd/system/docker.socket

添加内容如下：

[Unit]
Description=Docker Socket for the API
PartOf=docker.service

[Socket]
ListenStream=/var/run/docker.sock
SocketMode=0660
SocketUser=root
SocketGroup=docker

[Install]
WantedBy=sockets.target
Then run:

保存退出，执行如下动作：

    systemctl daemon-reload
    systemctl start docker