title: Specify Dockerfile for gcloud build submit
body: <p>I understand <code>gcloud</code> uses the Dockerfile specified in the root directory of the source (<code>.</code>) as in the command:
  <code>gcloud builds submit --tag gcr.io/[PROJECT_ID]/quickstart-image .</code></p>
  
  <p>but I am trying to specify the Dockerfile to use to build the image which I have not found any resource on how to do that, I don't know if that is possible.</p>