title: docker .env file reuse declared variables
body: <p>When I run docker-compose build I have a .env file to be used for building environment variables on my docker-machine like this:</p>
  
  <pre><code>COMPOSE_PROJECT_NAME=radar
  ENV=production
  DB_NAME=postgres
  DB_USER=postgres
  DB_PASS=sho1c2ov3Phezaimah7eb2Tii4ohkah8k
  DB_SERVICE=postgres
  DB_PORT=5432
  C_FORCE_ROOT="true"
  PGHOST=postgres
  PGDATABASE=postgres
  PGUSER=postgres
  PGPASSWORD=sho1c2ov3Phezaimah7eb2Tii4ohkah8k
  PGPORT=5432
  </code></pre>
  
  <p>If you have noticed the redundancy like the 'DB_NAME' and 'PGDATABASE' is the same.. Is there a way to avoid this?</p>