Add extra section on Gitlab Runners howto:

What does this Merge Request do?

After following this advanced instructions for gitlab runner the following error is present:

Could not load manifest from webpack-dev-server at http://localhost:3808/assets/webpack/manifest.json - is it running, and is stats-webpack-plugin loaded?

This MR will expand instructions on which sections of gitlab.yml need to be updated.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.