Default to gitlab container registry

What does this Merge Request do?

This MR switches GDK to use the Gitlab Container Registry CNG image by default.

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.

Backwards Compatibility

Users with container registry authorization enabled (default) will need to run gdk reconfigure with these changes, as the rootcertbundle needed to be moved to a location that the non-root user in the CNG image is able to access.