doctl: install shell completion

pkgs/development/tools/doctl/default.nix

@@ -1,4 +1,4 @@
-{ lib, buildGoPackage, fetchFromGitHub }:
+{ lib, buildGoPackage, fetchFromGitHub, installShellFiles }:
 
 buildGoPackage rec {
   pname = "doctl";
@@ -16,6 +16,16 @@ buildGoPackage rec {
     -X ${goPackagePath}.Label=release
   '';
 
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = ''
+    export HOME=$(mktemp -d) # attempts to write to /homeless-shelter
+    for shell in bash fish zsh; do
+      $bin/bin/doctl completion $shell > doctl.$shell
+      installShellCompletion doctl.$shell
+    done
+  '';
+
   src = fetchFromGitHub {
     owner = "digitalocean";
     repo = "doctl";