Adding ec2-user to docker group

Question

I want to install docker on my ec2 instance.

sudo yum install docker -y

I came to know that this command automatically creates a group 'docker' which has root privileges by default.so I add my ec2-user to this group to execute commands without 'sudo'.

sudo usermod -aG docker ec2-user

Now this means ec2-user has root privileges But if I want to start the docker service,why should I use

sudo systemctl start docker

instead of simply

systemctl start docker

Above command is giving me an error:

Failed to start docker.service: The name org.freedesktop.PolicyKit1 was not provided by any .service files See system logs and 'systemctl status docker.service' for details.

Please help!