Docker example should work on Darwin

[avdv]

Is your feature request related to a problem? Please describe.
The docs mention:

rules_haskell/docs/haskell-use-cases.rst

Lines 1203 to 1206 in 8a59b40

	*n.b.* Due to the `current inability`_ of Nix to be used on macOS (darwin) for building Docker images, it's currently
	not possible to build Docker images for Haskell binaries as above using ``rules_docker`` and Nixpkgs on macOS.
	.. _current inability: https://github.com/NixOS/nixpkgs/issues/16696

But the referenced issue is long resolved.

Describe the solution you'd like
Check if the example builds on Darwin and update the docs.

Describe alternatives you've considered

Additional context