Extend Docker install docs for Linux non-root users

[justb4]

Docker post-install steps in case of permission problems, even when using sudo.

See https://docs.docker.com/engine/install/linux-postinstall/ :

    If you don’t want to preface the docker command with sudo, create a Unix group called docker and add users to it. When the Docker daemon starts, it creates a Unix socket accessible by members of the docker group. On some Linux distributions, the system automatically creates this group when installing Docker Engine using a package manager. In that case, there is no need for you to manually create the group.
    
    To run Docker without root privileges, see [Run the Docker daemon as a non-root user (Rootless mode)](https://docs.docker.com/engine/security/rootless/).
    
    To create the docker group and add your user:
    
    Create the docker group.
    
    
     sudo groupadd docker

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