Move docker image build to buildx from docker-api gem for supporting multiarch builds

Currently, omnibus-gitlab uses the less actively maintained https://github.com/upserve/docker-api gem to build Docker images and push them to registries. For building multi-arch images, we are using docker buildx in other projects like CNG. This is not supported by this gem. We should move away from the gem to a solution that uses docker buildx to provide a foundation for building multiarch images.