There isn't an official Docker image for now as far as I know, so your best bet is to make an Ubuntu 14.04 Docker container and run the regular Sandstorm installer within it.

I agree it'd be great on NASs. I'm particularly excited about Synology ones since I know they have some x86-64 models. If you're interested in possibly helping out with that stuff, feel free to email community@sandstorm.io and I can see if there's a way we can give you a preview of that sooner rather than later.