Add postgres spec.

specs/postgres/runner

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+#!/usr/bin/env bash
+
+cp -r template/. "$OUT"

specs/postgres/template/.envrc

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+#!/usr/bin/env bash
+
+use flake

specs/postgres/template/README.md

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+# Postgres Flake Template
+
+This is a template for constructing a working environment for local
+[Postgres](https://www.postgresql.org/) (version 15.5) usage. [direnv](https://direnv.net/)
+can be used to a launch a dev shell upon entering this directory (refer to
+`.envrc`). Otherwise run via:
+```bash
+$> nix develop
+```
+
+## Quickstart
+
+To begin, create a new database:
+```bash
+pg_ctl initdb
+```
+If the flake's default `devShell` is loaded, this will create a database cluster
+at `$PWD/data`. To start the database, run the following:
+```bash
+pg_ctl start -o --unix_socket_directories="$PGDATA"
+```
+To shut the database down, run:
+```bash
+pg_ctl stop
+```
+You can also specify a different location for the database cluster using the
+`-D` option in each of the above commands.

specs/postgres/template/default.nix

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+(import
+  (
+    let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
+    fetchTarball {
+      url = lock.nodes.flake-compat.locked.url or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  { src = ./.; }
+).defaultNix

specs/postgres/template/flake.nix

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+{
+  description = ''
+    An opinionated postgres flake.
+
+    To generate a copy of this template elsewhere, run:
+    $> bootstrap postgres
+  '';
+
+  inputs = {
+    flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            postgresql_15
+          ];
+          shellHook = ''
+            # The server will try to use the data directory named by this
+            # environment variable whenever `-D` is not specified (for e.g.
+            # `postgres` and `pg_ctl`).
+            # https://www.postgresql.org/docs/15/server-start.html
+            export PGDATA="$PWD/data"
+          '';
+        };
+      });
+}