feat: nix flake for building guestbook

flake.nix

@@ -0,0 +1,26 @@
+{
+  description = "Guestbook";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    crane.url = "github:ipetkov/crane";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, crane, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        craneLib = crane.mkLib pkgs;
+        guestbook = craneLib.buildPackage {
+          src = craneLib.cleanCargoSource ./.;
+          buildInputs = with pkgs; [ openssl ];
+          nativeBuildInputs = with pkgs; [ pkg-config ];
+        };
+      in {
+        packages.default = guestbook;
+        devShells.default = craneLib.devShell {
+          packages = with pkgs; [ cargo rustc rust-analyzer ];
+        };
+      });
+}