title: Dockerfile: Is there any way to read variables from .env file
body: <p>I want to read the <code>PORT</code> variable inside Dockerfile which is defined in <code>.env</code> file. 
  Is there any way to do this?</p>
  
  <p>This is my Dockerfile:</p>
  
  <pre><code>FROM node:11-alpine
  
  RUN mkdir -p /usr/src/app
  
  WORKDIR /usr/src/app
  
  ENV PORT=3000
  
  COPY . .
  
  RUN npm install
  
  EXPOSE 3000
  
  CMD ["npm", "run", "start"]
  </code></pre>