title: docker - postgres upgrade 11 to 12 postmaster issue
body: <p><br >
  I used this <a href="https://github.com/tianon/docker-postgres-upgrade" rel="nofollow noreferrer">https://github.com/tianon/docker-postgres-upgrade</a> to upgrade postgres from 11 to 12 version &amp; facing issue while upgrade, got error - "postmaster servicing the old cluster"</p>
  
  <p>Here using docker container for 11 &amp; 12 in separately, also scale down docker service before that upgarde, but not sure why this postmaster issue came &amp; how to fix this.??  </p>
  
  <pre><code>docker run --rm \
      -v aip-pgs-data:/var/lib/postgresql/$OLD/data \
      -v aip-pg12-data:/var/lib/postgresql/$NEW/data \
      "tianon/postgres-upgrade:$OLD-to-$NEW"
  </code></pre>
  
  <p>Above docker run cmd part of logs:-- </p>
  
  <pre><code>Success. You can now start the database server using:
  
      pg_ctl -D /var/lib/postgresql/12/data -l logfile start
  
  
  There seems to be a postmaster servicing the old cluster.
  Please shutdown that postmaster and try again.
  Failure, exiting
  0a5839ad7309d6256510fe89513774a16c4f26ec6a827f9f0089fd8cc10254cb
  </code></pre>
  
  <p>Thank you in advance. </p>