By default, all tenant CI/CD runs are executed in a docker image specified by the administrator. This image would typically have the most common packages for your organization. But a user can bring his own builder image and specify the same. The image should have the Katkit agent that can be copied from the default builder image.
By default, Katkit will run the builder containers in a separate set of hosts, but the user can also choose to run the build container in the same tenant hosts which is being tested.