terraform-backend-git: install shell completions

pkgs/applications/networking/cluster/terraform-backend-git/default.nix

@@ -1,6 +1,7 @@
 { lib
 , buildGoModule
 , fetchFromGitHub
+, installShellFiles
 }:
 
 buildGoModule rec {
@@ -16,8 +17,19 @@ buildGoModule rec {
 
   vendorHash = "sha256-Y/4UgG/2Vp+gxBnGrNpAgRNfPZWJXhVo8TVa/VfOYt0=";
 
+  nativeBuildInputs = [
+    installShellFiles
+  ];
+
   ldflags = [ "-s" "-w" ];
 
+  postInstall = ''
+    installShellCompletion --cmd terraform-backend-git \
+      --bash <($out/bin/terraform-backend-git completion bash) \
+      --fish <($out/bin/terraform-backend-git completion fish) \
+      --zsh <($out/bin/terraform-backend-git completion zsh)
+  '';
+
   meta = with lib; {
     description = "Terraform HTTP Backend implementation that uses Git repository as storage";
     homepage = "https://github.com/plumber-cd/terraform-backend-git";