title: docker build command with variable in gitlab CI
body: <p>I have following docker command in gitlab CI file <code>.gitlab-ci.yml</code></p>
  <pre><code>docker build --pull -t &quot;${CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG}&quot; .
  </code></pre>
  <p>getting following error after build stage:</p>
  <blockquote>
  <p>invalid argument &quot;.&quot; for &quot;-t, --tag&quot; flag: invalid reference format
  See 'docker build --help'.</p>
  </blockquote>
  <p>any help , how to pass the variables ? Thanks</p>