Improve documentation on how to provide an image for Container Scanning tool

Problem to solve

From current Container Scanning example docs, it's not clear how and where to get the image for the tool to use and scan.

Proposal

Add a couple of lines to the example before the Container Scanning job definition:

• how to spin up a GitLab Container Registry (this could be just a link to the Registry docs)
• how to build an image and push it to the Registry

Update the job definition in the example:

• how to refer to the image correctly in the job definition in order to get it pulled and scanned

Who can address the issue

~Secure team

Product

• Release Post - Not Needed