Add option to build docker image

This commit adds dockerbuild to flakes.nix:

```
nix build .#headscale-docker
```

This uses the Nix infra to build and _does not_ use Dockerfile.

It currently works on Linux (no macOS)

flake.nix

@@ -84,6 +84,16 @@
               buf
               clang-tools # clang-format
             ];
+
+
+          # Add entry to build a docker image with headscale
+          # caveat: only works on Linux
+          headscale-docker = pkgs.dockerTools.buildLayeredImage {
+            name = "headscale";
+            tag = "latest";
+            contents = [ pkgs.${system}.headscale ];
+            config.Entrypoint = [ (pkgs.${system}.headscale + "/bin/headscale") ];
+          };
         in
         rec {
           # `nix develop`
@@ -92,6 +102,8 @@
           # `nix build`
           packages = with pkgs; {
             inherit headscale;
+            inherit headscale-docker;
+
           };
           defaultPackage = pkgs.headscale;
 
@@ -101,5 +113,6 @@
           };
           defaultApp = apps.headscale;
 
+
         });
 }