add a `flake.nix` for Nix builds

build with `nix build`; enter a dev environment with `nix develop`
wherein you can `cargo build`.

flake.lock

@@ -0,0 +1,42 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1659877975,
+        "narHash": "sha256-zllb8aq3YO3h8B/U0/J1WBgAL8EX5yWf5pMj3G0NAmc=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "c0e246b9b83f637f4681389ecabcb2681b4f3af0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1666164185,
+        "narHash": "sha256-5v+YB4ijeUfg5LCz9ck4gIpCPhIS+qn02OyPJO48bCE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c5203abb1329f7ea084c04acda330ca75d5b9fb5",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-22.05",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,42 @@
+{
+  description = "A simple log in screen for use on touch screens.";
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-22.05";
+    flake-utils.url = github:numtide/flake-utils;
+  };
+
+  outputs = { self, nixpkgs, flake-utils  }:
+    with flake-utils.lib; eachSystem allSystems (system:
+    let
+      pkgs = import nixpkgs { inherit system; };
+      buildInputs = with pkgs; [ gtk3 libhandy_0 lightdm ];
+      nativeBuildInputs = with pkgs; [ ];
+    in
+    rec {
+      packages = {
+        # docs: <nixpkgs>/doc/languages-frameworks/rust.section.md
+        # docs: https://github.com/oxalica/rust-overlay
+        lightdm-mobile-greeter = pkgs.rustPlatform.buildRustPackage {
+          pname = "lightdm-mobile-greeter";
+          version = "6";
+          inherit buildInputs nativeBuildInputs;
+          src = ./.;
+          cargoLock = {
+            lockFile = ./Cargo.lock;
+            outputHashes = {
+              "light-dm-sys-0.0.1" = "sha256-5o/DZqP6j6xRwf+YmrtQTpcg4PhTvQ0hKC1WauLdqzc=";
+            };
+          };
+        };
+      };
+      defaultPackage = packages.lightdm-mobile-greeter;
+
+      devShells.default = with pkgs; mkShell {
+        # Allow cargo to download crates.
+        SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";
+
+        # include both build and runtime deps in the shell so once can build and run the result.
+        buildInputs = buildInputs ++ nativeBuildInputs ++ [ cargo ];
+      };
+    });
+}