declarative cleanup

//assuming that docker-gc is installed on the agent

pipeline {
  agent {
    label 'mylabel'
  }
  stages {
    stage ("Build") {
      agent {
        dockerfile {
          filename 'Dockerfile'
          reuseNode true
        }
      }
      steps {
        echo "Foo"
      }
    }
  }
  post {
    always {
      sh 'docker-gc'
    }
  }
}