Add flake.nix for installation on NixOS

flake.nix

@@ -0,0 +1,168 @@
+{
+  description = "Local Docker Development DNS";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        packages = {
+          ldddns = pkgs.buildGo126Module {
+            pname = "ldddns";
+            version = self.shortRev or self.dirtyShortRev or "dev";
+
+            src = self;
+
+            # Relax the go directive to match the Go version available in nixpkgs.
+            prePatch = ''
+              substituteInPlace go.mod --replace-fail "go 1.26.3" "go 1.26"
+            '';
+
+            vendorHash = "sha256-FI7kSIn1QNDcEqDECiGNVLZ5z7LWPxxunK6eKH3D46Y=";
+
+            # Tests require /etc/protocols which is unavailable in the Nix sandbox.
+            doCheck = false;
+
+            env.CGO_ENABLED = 0;
+
+            ldflags = [
+              "-s"
+              "-w"
+              "-X main.version=${self.shortRev or self.dirtyShortRev or "dev"}"
+            ];
+
+            buildFlags = [ "-trimpath" ];
+
+            postInstall = ''
+              mv $out/bin/ldddns.arnested.dk $out/bin/ldddns
+            '';
+
+            meta = {
+              description = "Local Docker Development DNS";
+              homepage = "https://ldddns.arnested.dk";
+              license = pkgs.lib.licenses.mit;
+              maintainers = [ ];
+              platforms = pkgs.lib.platforms.linux;
+            };
+          };
+
+          default = self.packages.${system}.ldddns;
+        };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            go_1_26
+            gopls
+            goreleaser
+          ];
+        };
+      }
+    )
+    // {
+      nixosModules.default =
+        {
+          config,
+          lib,
+          pkgs,
+          ...
+        }:
+        let
+          cfg = config.services.ldddns;
+        in
+        {
+          options.services.ldddns = {
+            enable = lib.mkEnableOption "ldddns - Local Docker Development DNS";
+
+            package = lib.mkPackageOption pkgs "ldddns" {
+              default = self.packages.${pkgs.system}.ldddns;
+            };
+
+            hostnameLookup = lib.mkOption {
+              type = lib.types.listOf lib.types.str;
+              default = [
+                "env:VIRTUAL_HOST"
+                "containerName"
+              ];
+              description = "Methods for looking up hostnames for containers.";
+            };
+
+            ignoreDockerComposeOneoff = lib.mkOption {
+              type = lib.types.bool;
+              default = true;
+              description = "Whether to ignore docker-compose oneoff containers.";
+            };
+
+            gops = lib.mkOption {
+              type = lib.types.bool;
+              default = false;
+              description = "Whether to enable the Google gops diagnostics agent.";
+            };
+          };
+
+          config = lib.mkIf cfg.enable {
+            systemd.services.ldddns = {
+              description = "Local Docker Development DNS";
+              documentation = [ "https://ldddns.arnested.dk" ];
+              bindsTo = [
+                "docker.service"
+                "avahi-daemon.service"
+              ];
+              after = [
+                "docker.service"
+                "avahi-daemon.service"
+              ];
+              wantedBy = [ "docker.service" ];
+
+              environment = {
+                LDDDNS_HOSTNAME_LOOKUP = lib.concatStringsSep "," cfg.hostnameLookup;
+                LDDDNS_IGNORE_DOCKER_COMPOSE_ONEOFF =
+                  if cfg.ignoreDockerComposeOneoff then "true" else "false";
+                LDDDNS_GOPS = if cfg.gops then "true" else "false";
+              };
+
+              serviceConfig = {
+                Type = "notify";
+                ExecStart = "${cfg.package}/bin/ldddns start";
+                Restart = "on-failure";
+                SupplementaryGroups = [ "docker" ];
+                CapabilityBoundingSet = "";
+                DevicePolicy = "closed";
+                IPAddressDeny = "any";
+                LockPersonality = true;
+                MemoryDenyWriteExecute = true;
+                NoNewPrivileges = true;
+                PrivateDevices = true;
+                PrivateNetwork = true;
+                PrivateUsers = true;
+                ProtectClock = true;
+                ProtectControlGroups = true;
+                ProtectHome = true;
+                ProtectHostname = true;
+                ProtectKernelLogs = true;
+                ProtectKernelModules = true;
+                ProtectKernelTunables = true;
+                RestrictAddressFamilies = [ "AF_UNIX" ];
+                RestrictNamespaces = true;
+                RestrictRealtime = true;
+                SystemCallArchitectures = "native";
+                SystemCallErrorNumber = "EPERM";
+                SystemCallFilter = [ "@system-service" ];
+                UMask = "0777";
+              };
+            };
+          };
+        };
+    };
+}