CNG: Use buildx for building and tagging Docker images

In #1314 (closed) we concluded that we use BuildKit to support building multi-architecture Docker images and use buildx as interact with it.

The first step is to use docker buildx commands in build.sh and replace the existing docker CLI commands, in particular docker build and possibly docker tag with it.

We should attempt to eliminate docker tag and use an alternative solution, for example tagging at build time, to support multi-architecture manifests.