I don't know for sure, but I would image it has something to do with wanting to make a unified solution with how to manage things, but I see a lot of great options, such as setting up a free GitLab pipeline to build and push your image. You don't even have to use Docker with kaniko, if you want a Kubernetes-native image builder and there are great registries that can be deployed in Kubernetes like Harbor, with automated security scanning. This can all be done in GitLab as well with paid features. I also recommend checking out building and deploying rootless containers for builds.