解决：Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

CentOS 7 安装完 docker，所有 docker 命令都有这个提示

Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

解决办法：
编辑 /etc/docker/daemon.json，如果没有该文件就新建

 vim /etc/docker/daemon.json 

内容为：

{
 "registry-mirrors": ["https://b9pmyelo.mirror.aliyuncs.com"]
}

重启docker

systemctl restart docker.service