Fall back to template default if we don't know anything about Docker

src/briefcase/platforms/linux/system.py

@@ -583,9 +583,13 @@ def output_format_template_context(self, app: AppConfig):
 
         # Use the non-root user if Docker is not mapping usernames. Also use a non-root
         # user if we're on macOS; user mapping doesn't alter Docker operation, but some
-        # packaging tools (e.g., Arch's makepkg) don't like running as root.
+        # packaging tools (e.g., Arch's makepkg) don't like running as root. If we're
+        # not using Docker, this will fall back to the template default, which should be
+        # enabling the root user. This might cause problems later, but it's part of a
+        # much bigger "does the project need to be updated in light of configuration
+        # changes" problem.
         try:
-            context["use_non_root_user"] = self.use_docker and (
+            context["use_non_root_user"] = (
                 self.tools.host_os == "Darwin" or not self.tools.docker.is_user_mapped
             )
         except AttributeError:

tests/platforms/linux/system/test_create.py

@@ -120,6 +120,9 @@ def test_output_format_template_context(
 
 def test_output_format_template_context_no_docker(create_command, first_app_config):
     """If not using Docker, `use_non_root_user` default in template is used."""
+    # Mock the host to Linux to avoid flagging any "always use non-root user on macOS" logic.
+    create_command.tools.host_os = "Linux"
+
     # Add some properties defined in config finalization
     first_app_config.python_version_tag = "3.X"
     first_app_config.target_image = "somevendor:surprising"