Basics for building PostgREST with Nix (#1489)

* Add GHC options used with stack to postgrest.cabal

* Add a Nix derivation for just the executable.

nix/README.md

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+# Nix development and build environment
+
+With Nix it's possible to quickly and reliably recreate the full environments
+for developing, testing and building PostgREST.
+
+## Getting started with Nix
+
+You'll need to [get Nix](https://nixos.org/download.html). The installer will
+create your Nix store in the `/nix/` directory, where all build artifacts and
+their dependencies will be stored. It will also link the Nix executables like
+`nix-env`, `nix-build` and `nix-shell` into your PATH. Nix will manage all
+other PostgREST dependencies from here on out. To clean up older build
+artifacts from the `/nix/store`, you can run `nix-collect-garbage`.
+
+## Building PostgREST
+
+To build PostgREST from your local checkout of the repository, run:
+
+```bash
+nix-build --attr postgrest
+
+```
+
+This will create a `result` directory that contains the PostgREST binary at
+`result/bin/postgrest`. The `--attr` parameter (or short: `-A`) tells Nix to
+build the `postgrest` attribute from the Nix expression it finds in our
+`default.nix` (see below for details). Nix will take care of getting the right
+GHC version and all the build dependencies.
+
+## Developing
+
+A development environment for PostgREST is available with `nix-shell`. The
+following command will put you into a new shell that has GHC and Cabal on the
+PATH:
+
+```bash
+nix-shell
+
+```
+
+Within `nix-shell`, you can run Cabal commands as usual.
+
+## Tour
+
+The following is not required for working on PostgREST with Nix, but it will
+give you some more background and details on how it works.
+
+### `default.nix`
+
+[`default.nix`](../default.nix) is our 'respository expression' that pulls all
+the pieces that we define with Nix together. It returns a set (like a dict in
+other programming languages), where each attribute is a derivation that Nix
+knows how to build, like the `postgrest` attribute from earlier.
+
+Internally, our `default.nix` uses the `pkgs.callPackage` function to import
+the modules that we defined in the `nix` directory. It automatically passes the
+arguments those modules require if they are available in `pkgs` (this means
+that `pkgs` is defined in terms of itself, better not to think too much about
+that).
+
+We also use `default.nix` to load our pinned version of the `nixpkgs`
+repository. This set of packages will always be the same, independently from
+where or when you use it. The pinned version can be upgraded with the small
+`nixpkgs-upgrade` utility. Running `nixpkgs-upgrade > nix/nixpkgs-version.nix`
+in `nix-shell` will upgrade the pinned version to the latest `nixpkgs-unstable`
+version.
+
+### `shell.nix`
+
+[`shell.nix`](../shell.nix) defines an environment in which PostgREST can be
+built and developed. It extends the build enviroment from our `postgrest`
+attribute with useful utilities that will be put on the PATH in `nix-shell`.
+
+### `nix/overlays`
+
+Our overlays to the Nix package set are defined here. They allow us to tweak our
+`pkgs` in `default.nix` by adding new packages or overriding existing ones.

nix/nixpkgs-upgrade.nix

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+{ curl
+, jq
+, writeShellScriptBin
+, nix
+}:
+# Utility script for pinning the latest unstable version of Nixpkgs.
+
+# Instead of pinning Nixpkgs based on the huge Git repository, we reference a
+# specific tarball that only contains the source of the revision that we want
+# to pin.
+let
+  name =
+    "nixpkgs-upgrade";
+
+  refUrl =
+    https://api.github.com/repos/nixos/nixpkgs/git/ref/heads/nixpkgs-unstable;
+
+  githubV3Header =
+    "Accept: application/vnd.github.v3+json";
+
+  tarballUrlBase =
+    https://github.com/nixos/nixpkgs/archive/;
+in
+writeShellScriptBin name
+  ''
+    set -euo pipefail
+
+    commitHash="$(${curl}/bin/curl "${refUrl}" -H "${githubV3Header}" | ${jq}/bin/jq -r .object.sha)"
+    tarballUrl="${tarballUrlBase}$commitHash.tar.gz"
+    tarballHash="$(${nix}/bin/nix-prefetch-url --unpack "$tarballUrl")"
+    currentDate="$(date --iso)"
+
+    cat << EOF
+    # Pinned version of Nixpkgs, generated with ${name}.
+    {
+      date = "$currentDate";
+      rev = "$commitHash";
+      tarballHash = "$tarballHash";
+    }
+    EOF
+  ''

nix/nixpkgs-version.nix

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+# Pinned version of Nixpkgs, generated with nixpkgs-upgrade.
+{
+  date = "2020-04-19";
+  rev = "10100a97c8964e82b30f180fda41ade8e6f69e41";
+  tarballHash = "011f36kr3c1ria7rag7px26bh73d1b0xpqadd149bysf4hg17rln";
+}

nix/overlays/gitignore.nix

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+self: super:
+# Overlay that adds the `gitignoreSource` function from Hercules-CI.
+# This function is useful for filtering which files are added to the Nix store.
+# See: https://github.com/hercules-ci/gitignore.nix
+
+# To update to a newer revision, the simplest way is to add a new commit hash
+# from GitHub under `rev` and to then add the hash that Nix suggests on first
+# use.
+{
+  gitignoreSource =
+    let
+      gitignoreSrc = super.fetchFromGitHub {
+        owner = "hercules-ci";
+        repo = "gitignore";
+        rev = "2ced4519f865341adcb143c5d668f955a2cb997f";
+        sha256 = "sha256:0fc5bgv9syfcblp23y05kkfnpgh3gssz6vn24frs8dzw39algk2z";
+      };
+    in (super.callPackage gitignoreSrc {}).gitignoreSource;
+}