nixos/virtualisation/podman: add shell completions for podman dockerCompat

This can fix shell completion for `podman.dockerCompat`.

nixos/modules/virtualisation/podman/default.nix

@@ -10,11 +10,13 @@ let
   json = pkgs.formats.json { };
 
   inherit (lib) mkOption types;
+  inherit (pkgs) stdenv;
 
   # Provides a fake "docker" binary mapping to podman
   dockerCompat =
     pkgs.runCommand "${cfg.package.pname}-docker-compat-${cfg.package.version}"
       {
+        nativeBuildInputs = [ pkgs.installShellFiles ];
         outputs = [
           "out"
           "man"
@@ -31,7 +33,14 @@ let
           basename=$(basename $f | sed s/podman/docker/g)
           ln -s $f $man/share/man/man1/$basename
         done
-      '';
+      ''
+    + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+      export HOME=$(mktemp -d) # work around `docker <cmd>`
+      installShellCompletion --cmd docker \
+        --bash <($out/bin/docker completion bash) \
+        --zsh <($out/bin/docker completion zsh) \
+        --fish <($out/bin/docker completion fish)
+    '';
 
 in
 {