Move build/CNG to cloud-native/images
With &12 we are moving several projects, including this one, to https://gitlab.com/gitlab-org/cloud-native.
The final project path should be https://gitlab.com/gitlab-org/cloud-native/images.
Note that we have https://gitlab.com/gitlab-org/build/CNG-mirror mirroring this repository. We can decide closer to implementation whether we'd like to tackle that as part of this issue or as a separate issue.