nixos/amazon-init: add user-data shell script support

nixos/modules/virtualisation/amazon-init.nix

@@ -16,6 +16,16 @@ let
 
     userData=/etc/ec2-metadata/user-data
 
+    # Check if user-data looks like a shell script and execute it with the
+    # runtime shell if it does. Otherwise treat it as a nixos configuration
+    # expression
+    if IFS= LC_ALL=C read -rN2 shebang < $userData && [ "$shebang" = '#!' ]; then
+      # NB: we cannot chmod the $userData file, this is why we execute it via
+      # `pkgs.runtimeShell`. This means we have only limited support for shell
+      # scripts compatible with the `pkgs.runtimeShell`.
+      exec ${pkgs.runtimeShell} $userData
+    fi
+
     if [ -s "$userData" ]; then
       # If the user-data looks like it could be a nix expression,
       # copy it over. Also, look for a magic three-hash comment and set