chore: use nix-direnv

.gitignore

@@ -44,3 +44,7 @@ firebase-credentials.json
 
 ## Binaries ##
 bin
+
+## Environment ##
+.direnv
+.envrc

flake.nix

@@ -0,0 +1,37 @@
+{
+  description = "PND backend dev environment";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      goVersion = 21; # Change this to update the whole stack
+      overlays = [ (final: prev: { go = prev."go_1_${toString goVersion}"; }) ];
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forEachSupportedSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
+        pkgs = import nixpkgs { inherit overlays system; };
+      });
+    in
+    {
+      devShells = forEachSupportedSystem ({ pkgs }: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [
+            # go (specified by overlay)
+            go_1_21
+
+            # goimports, godoc, etc.
+            gotools
+
+            # https://github.com/mvdan/gofumpt
+            gofumpt
+
+            # https://github.com/golangci/golangci-lint
+            golangci-lint
+
+            # https://github.com/golang-migrate/migrate
+            go-migrate
+          ];
+        };
+      });
+    };
+}