Add flake build file

This commit adds a flake.nix build file, it can be used for three
things:

Build `headscale` from local or straight from git:

nix build
or
nix build github:juanfont/headscale

Run and Build `headscale` from local or straight from git:

nix run
or
nix run github:juanfont/headscale

Set up a development environment including all our tools,
- linters
- protobuf tooling
- compilers

nix develop

flake.nix

@@ -0,0 +1,105 @@
+{
+  description = "headscale - Open Source Tailscale Control server";
+
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    {
+      overlay = final: prev:
+        let
+          pkgs = nixpkgs.legacyPackages.${prev.system};
+        in
+        rec {
+          golines =
+            pkgs.buildGoModule rec {
+              pname = "golines";
+              version = "0.8.0";
+
+              src = pkgs.fetchFromGitHub {
+                owner = "segmentio";
+                repo = "golines";
+                rev = "v${version}";
+                sha256 = "sha256-W4vN3rGKyX43HZyjjVUKnR4Fy0LfYqVf6h7wIYO0U50=";
+              };
+
+              vendorSha256 = "sha256-ZHL2hQnJXpZu42hQZzIIaEzINSS+5BOL9dxAVCp0nMs=";
+
+              nativeBuildInputs = [ pkgs.installShellFiles ];
+            };
+
+          protoc-gen-grpc-gateway =
+            pkgs.buildGoModule rec {
+              pname = "grpc-gateway";
+              version = "2.8.0";
+
+              src = pkgs.fetchFromGitHub {
+                owner = "grpc-ecosystem";
+                repo = "grpc-gateway";
+                rev = "v${version}";
+                sha256 = "sha256-8eBBBYJ+tBjB2fgPMX/ZlbN3eeS75e8TAZYOKXs6hcg=";
+              };
+
+              vendorSha256 = "sha256-AW2Gn/mlZyLMwF+NpK59eiOmQrYWW/9HPjbunYc9Ij4=";
+
+              nativeBuildInputs = [ pkgs.installShellFiles ];
+
+              subPackages = [ "protoc-gen-grpc-gateway" "protoc-gen-openapiv2" ];
+            };
+
+          headscale =
+            pkgs.buildGo117Module rec {
+              pname = "headscale";
+              version = "dev";
+              src = pkgs.lib.cleanSource self;
+
+              # When updating go.mod or go.sum, a new sha will need to be calculated,
+              # update this if you have a mismatch after doing a change to thos files.
+              vendorSha256 = "sha256-XzcTErmY/jM73nGzH3R0+5lIqJ8tT1lFyQilwJpqBlo=";
+
+              ldflags = [ "-s" "-w" "-X github.com/juanfont/headscale/cmd/headscale/cli.Version=v${version}" ];
+            };
+        };
+    } // flake-utils.lib.eachDefaultSystem
+      (system:
+        let
+          pkgs = import nixpkgs {
+            overlays = [ self.overlay ];
+            inherit system;
+          };
+          buildDeps = with pkgs; [ git go_1_17 gnumake ];
+          devDeps = with pkgs;
+            buildDeps ++ [
+              golangci-lint
+              golines
+              nodePackages.prettier
+
+              # Protobuf dependencies
+              protobuf
+              protoc-gen-go
+              protoc-gen-go-grpc
+              protoc-gen-grpc-gateway
+              buf
+              clang-tools # clang-format
+            ];
+        in
+        rec {
+          # `nix develop`
+          devShell = pkgs.mkShell { buildInputs = devDeps; };
+
+          # `nix build`
+          packages = with pkgs; {
+            inherit headscale;
+          };
+          defaultPackage = pkgs.headscale;
+
+          # `nix run`
+          apps.headscale = flake-utils.lib.mkApp {
+            drv = packages.headscale;
+          };
+          defaultApp = apps.headscale;
+
+        });
+}