title: docker-compose secrets without swarm
body: <p>I don't want to use docker secrets with swarm and I discovered that it's possible to do that.
  Basically docker just mounts /run/secrets inside docker container, but when I enter the newly built docker container and do <code>echo $POSTGRES_PASSWORD_FILE</code> I get the path to my secret file.</p>
  
  <pre><code>root@94a0f092eeb1:/# echo $POSTGRES_PASSWORD_FILE
  /run/secrets/db_password
  </code></pre>
  
  <p>Here is my <code>docker-compose.yml</code> file</p>
  
  <pre><code>version: '3.1'
  services:
      postgres:
          image: postgres:9.4
          container_name: postgres
          environment:
              POSTGRES_USER: "db_user"
              POSTGRES_PASSWORD_FILE: /run/secrets/db_password
              POSTGRES_DB: "my_db"
          secrets:
            - db_password
          volumes:
              - ./postgres:/var/lib/postgresql/data
          expose:
              - 5432
  secrets:
     db_password:
       file: ./POSTGRES_PASSWORD.txt
  </code></pre>
  
  <p>Is my password set correctly/ Is there something wrong with my file? </p>