docs: update nix files

flake.nix

@@ -0,0 +1,75 @@
+{
+  description = "Flake for terraform-gcp-meshplatform";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-24.05";
+  };
+
+  outputs = { self, nixpkgs }:
+
+  let
+    # These tools are pre-installed in github actions, so we can save the time for installing them.
+    github_actions_preinstalled = pkgs:
+      with pkgs;
+      [
+        awscli2
+        (azure-cli.withExtensions [ azure-cli.extensions.account ])
+        nodejs
+      ];
+
+    # core packages required in CI and not preinstalled in github actions
+    core_packages = pkgs:
+      let
+        tofu_terraform =
+          pkgs.stdenv.mkDerivation {
+            name = "tofu-terraform";
+            phases = [ "installPhase" ];
+            installPhase = ''
+              mkdir -p $out/bin
+              echo '#!/usr/bin/env sh' > $out/bin/terraform
+              echo 'tofu "$@"' >> $out/bin/terraform
+              chmod +x $out/bin/terraform
+            '';
+          };
+      in
+      with pkgs;
+      [
+        opentofu
+        terragrunt
+        tflint
+        tfupdate
+        terraform-docs
+        tofu_terraform
+        pre-commit
+      ];
+
+    importNixpkgs = system: import nixpkgs { inherit system; };
+
+    defaultShellForSystem = system:
+      let
+        pkgs = importNixpkgs system;
+      in {
+        default = pkgs.mkShell {
+          name = "terraform-gcp-meshplatform";
+          packages = (github_actions_preinstalled pkgs) ++ (core_packages pkgs);
+        };
+      };
+
+  in {
+    devShells = {
+      aarch64-darwin = defaultShellForSystem "aarch64-darwin";
+      x86_64-darwin = defaultShellForSystem "x86_64-darwin";
+      x86_64-linux = defaultShellForSystem "x86_64-linux" // {
+        github_actions =
+          let
+            pkgs = importNixpkgs "x86_64-linux";
+          in
+          pkgs.mkShell {
+            name = "terraform-gcp-meshplatform";
+            packages = (core_packages pkgs);
+          };
+      };
+    };
+  };
+}
+