错误：
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
如果在Centos7上执行：
systemctl daemon-reload
sudo service docker restart
报出新的错误：
Redirecting to /bin/systemctl restart docker.service
Failed to restart docker.service: Unit not found.
解决方式：
缺少了如下的文件：/usr/lib/systemd/system/docker.socket
[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
添加后执行
systemctl daemon-reload systemctl start docker.socket systemctl start docker
