Jenkinsfile docker run outputting Jenkins user UID

[nwa...@rednovalabs.com]

We're using Jenkins 2 on an Ubuntu 14.04 box and I'm trying out the new Jenkinsfile stuff...  something like this:

node('docker') {

  stage "Prepare environment"

    checkout scm

    docker.image('geerlingguy/docker-ubuntu1604-ansible:latest').inside{

      stage('Syntax Check') {

        sh 'whoami'

        sh 'echo $HOME'

        sh 'ansible-playbook $PWD/tests/test.yml --syntax-check'

      }

    }

}

When Jenkins goes to run the "whoami" command, I can see the command it did was:

docker run -t -d -u 1003:1003

The user ID of the Jenkins user on this machine is 1003, but I'm not sure why it would be outputting that by default.  Is there a way to turn it off?