Use pinned nixpkgs (#117)
* pin nixpkgs
* use pinned nixpkgs
* use pinned nixpkgs for docker build
* use buildLayeredImage
* test dhall
* dhall channel
* fix??
* fix???
* fix????
* easy-dhall-nix
* fix?????
* pin dhall-yaml to shell, update xepkgs to use inherit
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