Resolve Docker image URI mismatch in container_scanning job

What does this merge request do and why?

This MR fixes a Docker image URI mismatch issue that has been causing the container_scanning job to fail (e.g. https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/4384336831).

Previously, we set the variable DOCKER_IMAGE in the container_scanning job. But, after looking at the available CI/CD variables in the Container Scanning docs, I found that the DOCKER_IMAGE variable no longer exists. Instead, we can use the CS_IMAGE variable which is a replacement of the DOCKER_IMAGE variable.

Closes #1840 (closed)

How to set up and validate locally

You can see a successful container_scanning job: https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/4388122164.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.