title: Add variable to dockerfile{} section in Jenkinsfile to enable docker_buildkit
body: <p>This is snipshet of my code:</p>
  
  <pre><code>   stage('tf run') {
    agent {
      dockerfile {
        additionalBuildArgs "${ADDITIONAL_BUILD_ARGS}"
        registryCredentialsId "${REGISTRY_CREDENTIALS}"
        registryUrl "${REGISTRY_URL}"
        reuseNode true
      }
    }
  </code></pre>
  
  <p>Now I want do add special variable DOCKER_BUILDKIT=1 when I run this peace of code.
  For example in a shell it should look like: "DOCKER_BUILDKIT=1 docker build ."
  I need it because I want to run my docker in docker_buildkit mode.
  How can I do this?</p>