Add trivy-kubernetes docker image

What does this MR do

• Adds a build folder for trivy-kubernetes docker image
• Extends Makefile with building and pushing trivy-kubernetes docker image
• Extends .gitlab-ci.yml with relevant jobs for building and pushing the trivy-kubernetes docker image.

Why do we do this

The intention is to use this wrapper Trivy image to scan namespaces in a K8S cluster. This image will send the scan report to the agent by an HTTP request.

For now we do not use any authentication in the HTTPs request. This will come later with gitlab-org/gitlab#431331 (closed). This is safe for now since no-one is using this image.

Related issues

• Send vulnerability report using a network reque... (gitlab-org/gitlab#407649 - closed) • Nick Ilieskou • 16.7
• Create a Trivy wrapper image (gitlab-org/gitlab#431320 - closed) • Nick Ilieskou • 16.7 • On track