title: Google Cloud Build Docker build-arg not respected
body: <p>I'm having a problem with Google Cloud Build where the docker build command doesn't seem to be accepting the build-arg option, even though this same command works as expected on local:</p>
  
  <p>Dockerfile:</p>
  
  <pre><code>ARG ASSETS_ENV=development
  RUN echo "ASSETS_ENV is ${ASSETS_ENV}"
  </code></pre>
  
  <p>Build Command: </p>
  
  <pre><code>docker build --build-arg="ASSETS_ENV=production" .
  </code></pre>
  
  <p>Result on local:</p>
  
  <pre><code>ASSETS_ENV is production
  </code></pre>
  
  <p>Result on Cloud Build:</p>
  
  <pre><code>ASSETS_ENV is development
  </code></pre>