Add watch mode for gitlab-ui

What does this Merge Request do and why?

Build and watch gitlab-ui when gitlab_ui is enabled in config

Adds a gitlab-ui service that runs yarn build --watch in the gitlab-ui directory. Aim is to use this in gitlab in gitlab!59605 (closed) - which is hopefully cheaper/easier than using yarn link which sometimes has confusing issues.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.