NixOS-based image support

[roberth]

Using NixOS inside a service is already possible, but not well integrated into Arion. A 'standard' interface seems helpful because it should make switching between init systems easier. For instance, it creates a standard way to add NixOS options to docker-compose services.

• Service-level option for NixOS configuration to be written by user
  • cast to list of modules
  • merge function is list concatenation
• Service-level option for evaluated NixOS configuration to be read by user or internal module
  • defined by calling pkgs.nixos function with these modules
• Internal option and module for nlewo's nix-container-images moved to S6 init system support #24
  • may or may not be able to use generalized call to NixOS
  • may need to provide extra module to nix-container-images in order to support useHostStore properly
• Internal option and module for services.systemd.*.runner-based containers

[roberth]

There doesn't seem to be that much we can do for systemd runner based containers. Promoting that approach also seems like a bad idea.