Add nix flake (#5198)

* Add flake

* Add gemset

* Add nix instructions to README

README.md

@@ -84,6 +84,23 @@ The complete [role description of a CASA volunteer](https://pgcasa.org/volunteer
 2. You can ask a [maintainer](https://github.com/rubyforgood/casa/wiki/Who's-who%3F) for permission to make a branch on this repo.
 3. You can also [create a fork on GitHub](https://docs.github.com/en/get-started/quickstart/fork-a-repo) and make a pull request from the fork.
 
+**Nix** 
+
+If you have [Nix](https://nixos.org) installed you can use
+the [flake.nix](flake.nix) configuration file located at the root of the
+project to build and develop within an environment without needing to install 
+`rvm`, `nodejs`, `yarn` or other tools separately. All you need is this file 
+to get the server up and running:
+
+1. Install [Nix](https://zero-to-nix.com/concepts/nix-installer)
+1. `cd` into casa
+1. `nix develop`
+1. `bundix` # to generate a a Gemfile lock that nix can understand
+1. `bundle install`
+
+Then you can setup the database and run the server.
+This will run on Linux and macOS.
+
 **Ruby**
 1. Install a ruby version manager: [rvm](https://rvm.io/) or [rbenv](https://github.com/rbenv/rbenv)
 1. when you cd into the project directory, let your version manager install the ruby version in `.ruby-version`. Right now that's Ruby 3.2.2

flake.nix

@@ -0,0 +1,97 @@
+{
+  description = "A Ruby dev environment for Casa Development";
+
+  nixConfig = {
+    extra-substituters = "https://nixpkgs-ruby.cachix.org";
+    extra-trusted-public-keys =
+      "nixpkgs-ruby.cachix.org-1:vrcdi50fTolOxWCZZkw0jakOnUI1T19oYJ+PRYdK4SM=";
+  };
+
+  inputs = {
+    nixpkgs.url = "nixpkgs";
+    ruby-nix = {
+      url = "github:inscapist/ruby-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    bundix = {
+      url = "github:inscapist/bundix/main";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    fu.url = "github:numtide/flake-utils";
+    bob-ruby = {
+      url = "github:bobvanderlinden/nixpkgs-ruby";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, fu, ruby-nix, bundix, bob-ruby }:
+    with fu.lib;
+    eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ bob-ruby.overlays.default ];
+        };
+        rubyNix = ruby-nix.lib pkgs;
+
+        gemset =
+          if builtins.pathExists ./gemset.nix then import ./gemset.nix else { };
+
+        # See available versions here: https://github.com/bobvanderlinden/nixpkgs-ruby/blob/master/ruby/versions.json
+        ruby = pkgs."ruby-3.2.2";
+
+        bundixcli = bundix.packages.${system}.default;
+      in rec {
+        inherit (rubyNix {
+          inherit gemset ruby;
+          name = "ruby-env-casa";
+          gemConfig = pkgs.defaultGemConfig // gemConfig;
+        })
+          env;
+
+        devShells = rec {
+          default = dev;
+          dev = pkgs.mkShell {
+            BUNDLE_PATH = "~/.gems";
+            shellHook = ''
+              export PS1='\n\[\033[1;34m\][💎:\w]\$\[\033[0m\] '
+
+              # Setup postgres database
+              export PGHOST=$HOME/postgres
+              export PGDATA=$PGHOST/data
+              export PGDATABASE=postgres
+              export PGLOG=$PGHOST/postgres.log
+
+              mkdir -p $PGHOST
+
+              if [ ! -d $PGDATA ]; then
+                initdb --auth=trust --no-locale --encoding=UTF8
+              fi
+
+              if ! pg_ctl status
+              then
+                pg_ctl start -l $PGLOG -o "--unix_socket_directories='$PGHOST'"
+              fi
+            '';
+
+            buildInputs = [
+              env
+              pkgs.bundix
+              pkgs.bundler-audit
+              pkgs.direnv
+              pkgs.git
+              pkgs.gnumake
+              pkgs.libpcap
+              pkgs.libpqxx
+              pkgs.libxml2
+              pkgs.libxslt
+              pkgs.nodejs-18_x
+              pkgs.pkg-config
+              pkgs.postgresql
+              pkgs.sqlite
+              pkgs.yarn
+            ];
+          };
+        };
+      });
+}