For nix, build from source?

I've been [investigating](https://github.com/eikek/docspell/compare/master...antifuchs:nix-restructure) what it would take to have a nix definition that makes the current source tree installable under nix (rather than referring to a previous release from any revision _after_ that release)... That semi-works, too - except, it fails when built in the nix sandbox (of course, as sbt wants to download stuff).

I think it would be really nice if we could have adjacent definitions for the set of dependencies in a way that allows nix to have them ready for sbt. I believe there are a few of those tools, and it would be really cool if we could integrate one of those, provided one of them works for docspell! (: