Add support for BuildKit when building images

[juanfont]

Docker/Mobi now has two builders: the classic one, and the one that comes under the buildx command ("BuildKit"). This new plugin adds better support for multi-platform images https://docs.docker.com/build/buildx/multiplatform-images/.

This small PR updates BuildOptions and docker.BuildImageOptions to include a new field Version, that is used to specify which builder should be used, as per https://github.com/docker/engine/blob/19.03/api/types/client.go#L185

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