Improve container scanning info in registry docs

What does this Merge Request do?

This MR documents using container scanning with a local Docker container registry with the following three different options:

1. Using an insecure registry
2. Using a secure registry, but without referencing the certificate
3. Using a secure registry and referencing the certificate

Related issues

#858 (closed)

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.