ci: enable multiplatform builds with docker buildx

* Build from the Dockerfile and copy the artifacts from the resulting
  image
* Use cross-compilation at build stage when building for a foreign architecture
* Use 'docker buildx build ...' in CI to enable multiplatform builds
  If buildx is not installed use:
    DOCKER_BUILDKIT=1 docker build ...
* Gilab runner must expose tag 'multiplatform' which means it has native
  amd64 arch + emulated arm64 support:
    https://github.com/tonistiigi/binfmt/tree/deploy/v7.0.0-28#installing-emulators
    $ docker run --privileged --rm tonistiigi/binfmt --install arm64