title: Where do $POSTGRES_PORT_5432_TCP_ADDR and $POSTGRES_PORT_5432_TCP_PORT come from?
body: <p>I am trying docker postgres by following the official postgres image on <a href="https://registry.hub.docker.com/_/postgres/" rel="noreferrer">https://registry.hub.docker.com/_/postgres/</a> . </p>
  
  <p>In the document it run the following commands and get it to work: </p>
  
  <pre><code>docker run --name some-postgres -e POSTGRES_PASSWORD=mysecretpassword -d postgres
  docker run -it --link some-postgres:postgres --rm postgres sh -c 'exec psql -h "$POSTGRES_PORT_5432_TCP_ADDR" -p "$POSTGRES_PORT_5432_TCP_PORT" -U postgres'
  </code></pre>
  
  <p>So my question is where do $POSTGRES_PORT_5432_TCP_PORT and $POSTGRES_PORT_5432_TCP_ADDR come from?</p>