add nix flake for dev environment

flake.nix

@@ -0,0 +1,49 @@
+{
+  inputs = {
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = inputs @ { self, ... }:
+    (inputs.flake-utils.lib.eachDefaultSystem (system:
+      let
+
+        pkgs = import inputs.nixpkgs {
+          inherit system;
+        };
+
+        ruby = pkgs.ruby_3_0;
+
+        bundix = pkgs.bundix.override {
+          bundler = pkgs.bundler.override {
+            ruby = pkgs.ruby_3_0;
+          };
+        };
+
+        rubyGems = pkgs.bundlerEnv {
+          name = "ruby-gems-env";
+          inherit ruby;
+          gemdir = ./.;
+        };
+
+        shellPkgs = [
+          bundix
+          pkgs.bundler
+          ruby
+          rubyGems
+        ] ++ (with pkgs; [
+          nodejs-18_x
+        ]);
+
+      in
+      rec {
+
+        devShells = {
+          default = pkgs.mkShell {
+            buildInputs = shellPkgs;
+            BUNDLE_FORCE_RUBY_PLATFORM = "true";
+          };
+        };
+
+      }));
+}