nix: Switch to ruby-nix dev shell

Creates a fully reproducible development shell with Gem versions derived
from Gemfile.lock.

_config.yml

@@ -40,6 +40,7 @@ exclude:
   - LICENSE*.txt
   - CNAME
   - flake.*
+  - gemset.nix
 
 plugins:
   - jekyll-feed

flake.nix

@@ -2,30 +2,63 @@
   description = "GitHub Pages";
 
   inputs.nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1.*.tar.gz"; # nixos-unstable
+
+  inputs.ruby-nix = {
+    url = "github:inscapist/ruby-nix";
+    inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  inputs.bundix = {
+    url = "github:inscapist/bundix/main";
+    inputs.nixpkgs.follows = "nixpkgs";
+  };
+
   inputs.flake-schemas.url = "https://flakehub.com/f/DeterminateSystems/flake-schemas/0.1.*.tar.gz";
 
-  outputs = { self, nixpkgs, flake-schemas }:
+  outputs = { self, nixpkgs, ruby-nix, bundix, flake-schemas }:
     let
       allSystems = [
         "x86_64-linux"
         "aarch64-darwin"
       ];
 
       forAllSystems = f: nixpkgs.lib.genAttrs allSystems (system: f {
+        # Current 'ruby' package version: 3.3.4
+        # Should match https://github.com/actions/jekyll-build-pages/blob/v1.0.13/Dockerfile
         pkgs = import nixpkgs { inherit system; };
       });
     in
     {
       inherit (flake-schemas) schemas;
 
-      devShells = forAllSystems ({ pkgs }: {
-        default = with pkgs; mkShell {
-          packages = [
-            ruby
-            rubyPackages.github-pages
-            rubyPackages.webrick # https://github.com/jekyll/jekyll/pull/8524
-          ];
-        };
-      });
+      devShells = forAllSystems ({ pkgs }:
+        let
+          ruby-env = (ruby-nix.lib pkgs {
+            name = "github-pages";
+            gemset = ./gemset.nix;
+          }).env;
+
+          bundix-cli = bundix.packages.${pkgs.system}.default;
+
+          bundleLock = pkgs.writeShellScriptBin "bundle-lock" ''
+            export BUNDLE_PATH=vendor/bundle
+            bundle lock
+          '';
+          bundleUpdate = pkgs.writeShellScriptBin "bundle-update" ''
+            export BUNDLE_PATH=vendor/bundle
+            bundle lock --update
+          '';
+        in
+        {
+          default = pkgs.mkShell {
+            buildInputs = [
+              ruby-env
+              bundix-cli
+              bundleLock
+              bundleUpdate
+            ];
+          };
+        }
+      );
     };
 }