docker helloworld

$ docker run helloworld

1. Firstly, check whether there is a user group for docker.
   $ sudo cat /etc/group | grep docker

2. If not, then create the docker group.
   $ sudo groupadd docker

3. Add your user to the docker group.
   $ sudo usermod -aG docker $USER

4. double check
   $ sudo cat /etc/group |grep docker

5. Restart docker
   $ sudo systemctl restart docker   or https://blog.csdn.net/u013392078/article/details/103800411

6. check below commands again.
   $ docker run helloworld

    

7. Reference: https://docs.docker.com/v17.09/engine/installation/linux/linux-postinstall/#manage-docker-as-a-non-root-user