title: how to pass variable to Dockerfile while using gcloud builds submit
body: <p>I'm quite new for pretty much everyhing.
  Docker.
  Gcloud.
  Gitlab :D</p>
  <p>I need to pass some configuration to my dockerfile.
  because angular part should be compiled with different configurations for each env.</p>
  <p>i intend to use --substitutions but it seems like i can use it for passing another variable other than:</p>
  <p>Only the following built-in variables can be specified with the --substitutions flag: REPO_NAME, BRANCH_NAME, TAG_NAME, REVISION_ID, COMMIT_SHA, SHORT_SHA.
  <a href="https://cloud.google.com/sdk/gcloud/reference/builds/submit#--substitutions" rel="nofollow noreferrer">https://cloud.google.com/sdk/gcloud/reference/builds/submit#--substitutions</a></p>
  <p>i used --set-env-vars but it is used for gcloud deploy.</p>
  <p>so i'm asking while using gcloud build submit how can I pass a key/value pair to my Dockerfile?</p>