title: Env file is not read when I use docker-compose -f in a different directory
body: <p>I have a problem with my Docker <code>.env</code> file. When I'm in my Docker folder (where I have the <code>docker-compose.yml</code> and <code>.env</code> file), the environment variables are read from <code>.env</code> fine. However, when I run <code>docker-compose -f ./docker/docker-compose.yml up --build -d</code> in a higher-level directory I  get warnings like:</p>
  
  <pre><code>The MYSQL_ROOT_PASSWORD variable is not set. Defaulting to a blank string. 
  </code></pre>
  
  <p>Has anyone encountered this problem? I tried setting the <code>env_file</code> property in <code>docker-compose.yml</code> and using other file encodings (I found this solution in Google) and still nothing.</p>
  
  <p>This is my config:</p>
  
  <pre><code>version: '2'
  
  services:
      mysql:
          container_name: mysql
          image: mysql
          volumes:
              - "./.data/db:/var/lib/mysql"
          environment:
              MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
              MYSQL_DATABASE: ${MYSQL_DATABASE}
              MYSQL_USER: ${MYSQL_USER}
              MYSQL_PASSWORD: ${MYSQL_PASSWORD}
  </code></pre>
  
  <p>.env file:</p>
  
  <pre><code># MySQL
  MYSQL_ROOT_PASSWORD=test
  MYSQL_DATABASE=test
  MYSQL_USER=test
  MYSQL_PASSWORD=test
  </code></pre>