RFC: use Nickel to generate Dockerfiles

[develop7]

This replaces Haskell-based generator with a set of files in Nickel.

Pros:

1. Minimal footprint — Nickel evaluator is a single binary in Rust, contrary to the Haskell toolchain. And blazing fast too.
2. Native types (contracts) — YAML would require JSON Schema and external validator, Nickel contracts are written in Nickel itself.
3. Gradual typing — start cheap with Strings and Numbers (or without types even, not a problem), harden requirements when needed.
4. Native string interpolation — the Dockerfile template (templating function) is also written in Nickel.
5. Lean but rich standard library — should cover Jinja functionality.

Cons: another programming language, albeit small & simple.

[jhrcek]

In principle I don't have objections.
I must admin I'm not familiar with nickel and in general expect that fewer people will be familiar with it than with haskell/yaml, I trust folks from Tweag develop technically robust implementation of a configuration language.