Docker: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock

I am new to docker. I just tried to use docker in my local machine(Ubuntu 16.04) with Jenkins.

I configured a new job with below pipeline script.

node {
    stage('Build') {
      docker.image('maven:3.3.3').inside {
        sh 'mvn --version'
      }
    }
}

But it fails with below error.