Provide container/VM-based ways to work with reflex

Nix does not work on certain platforms (at least: Windows and Arch Linux).  For users of those platforms, try-reflex isn't useful, so we should provide a docker image, an OVA file, or something like that for them to use.

* We already can build an OVA from reflex-platform by running `nix-build -A demoVM`