title: Gitlab execute stage conditionally
body: <p>There are 3 stages - build, test and deploy in <code>.gitlab-ci.yml</code>.</p>
  
  <p>A nightly regression test stage needs to be run - well <code>nightly</code> :)</p>
  
  <p>Here's the relevant <code>.gitlab-ci.yml</code> code:</p>
  
  <pre class="lang-rb prettyprint-override"><code>stages:
    - build
    - test
    - deploy
  
  build_project:
    stage: build
    script:
      - cd ./some-dir
      - build-script.sh
    except:
    - tags
  
  #Run this only when say variable 'NIGHTLY_TEST == True'. But HOW?
  nightly_regression_test_project:
    stage: test
    script:
      - cd ./some-dir
      - execute test-script
  </code></pre>
  
  <p>Tagging daily to <code>only</code> run <code>test</code> stage is not preferable.</p>
  
  <p>Any other idea?</p>