Docker初次安装运行出错：Cannot connect to the Docker daemon at ***/docker.sock. Is the docker daemon running?

报错信息如下：Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

原因未启动docker服务，解决方法：

sudo systemctl  start docker

另外docker命令执行时，需要sudo　权限。