Configure CNG build scripts to allow multi-architecture Docker build

We use BuildKit Kubernetes driver to run parallel native builds with docker buildx on CNG build fleet.

Configure buildx as such that it can run multi-architecture build. This will require adding additional build platform as well as supporting multi-architecture manifests.

For this purpose:

• Run docker buildx create with kuberentes driver and pass required driver options to set up the native builders.
• Pass platforms as build parameter to buildx build command. See --platform.
• Add required CI variables for the configuration above to work. For example KUBECONFIG file for the build fleet cluster and default value for PLATFORM.