title: Docker compose - secrets Additional property secrets is not allowed
body: <p>docker-compose --version
  docker-compose version 1.11.1, build 7c5d5e4</p>
  
  <p>I have secret 'my_secret_data' added to my swarm cluster:</p>
  
  <p>The start of my compose file looks like:</p>
  
  <pre><code>version: "3.1"
  secrets:
    my_secret_data:
      external: true
  services:
    master:
      image: jenkins-master
      secrets:
       - my_secret_data
      ports:
       - "8080:8080"
       - "50000:50000" 
  </code></pre>
  
  <p>'docker stack deploy' continually gives the error:</p>
  
  <p>secrets Additional property secrets is not allowed</p>
  
  <p>I have followed <a href="https://stackoverflow.com/questions/42139605/how-do-you-manage-secret-values-with-docker-compose-v3-1">how do you manage secret values with docker-compose v3.1?</a> to the letter as far as I can tell and have the correct versions installed but keep getting the above error. Any help greatly appreciated.</p>