This adds a new `imageStream` option that can be used in conjunction
with `pkgs.dockerTools.streamLayeredImage` so that the image archive
never needs to be materialized in the `/nix/store`. This greatly
improves the disk utilization for systems that use container images
built using Nix because they only need to store image layers instead of
the full image. Additionally, when deploying the new system and only
new layers need to be built/copied.
