title: Docker build showing invalid containerPort
body: <p>I need help in debugging this image build:
  My dockerfile is :</p>
  <pre><code>FROM node:latest
  WORKDIR /app
  COPY . .
  ENV PORT = 3000
  RUN npm install
  EXPOSE $PORT
  ENTRYPOINT [ &quot;node&quot;, &quot;app.js&quot; ]
  
  </code></pre>
  <p>and when I run <code>sudo docker build -t uddeshya/node1 .</code> to build the image, the log shows the following:</p>
  <pre><code>Sending build context to Docker daemon  2.009MB
  Step 1/7 : FROM node:latest
   ---&gt; dcda6cd5e439
  Step 2/7 : WORKDIR /app
   ---&gt; Using cache
   ---&gt; 9450405b180f
  Step 3/7 : COPY . .
   ---&gt; Using cache
   ---&gt; 91689830af35
  Step 4/7 : ENV PORT = 3000
   ---&gt; Using cache
   ---&gt; d99d55d0ae81
  Step 5/7 : RUN npm install
   ---&gt; Using cache
   ---&gt; dac4854ec168
  Step 6/7 : EXPOSE $PORT
  Invalid containerPort: =
  </code></pre>
  <p>The error point is at &quot;EXPOSE $PORT&quot;, how do I fix this?</p>