meta: support nix flake for buliding

As noted in [1] on Linux systems, it is not possible to build phoenixd due to a mismatch in the libc version.

A possible solution is to use Nix and build phoenixd inside the shell with all the dependencies. This way, the host machine can use the binary directly without downgrading libc, which can be dangerous.

[1] ACINQ#1 (comment)
Co-authored-by: Pavol Rusnak <pavol@rusnak.io>
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

flake.nix

@@ -0,0 +1,44 @@
+{
+  description = "Server equivalent of the popular Phoenix wallet";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-24.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ ];
+        };
+      in
+      {
+        packages = {
+          default = pkgs.gnumake;
+        };
+        formatter = pkgs.nixpkgs-fmt;
+
+        devShell = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            # build dependencies
+            sqlite
+            curl
+            jdk
+
+            pkg-config
+            ncurses
+            stdenv.cc.cc.lib
+
+            git
+          ];
+
+          shellHook = ''
+             # FIXME: this need to go in a build task
+            ./gradlew linuxX64DistZip
+          '';
+        };
+      }
+    );
+}