Disable building Arch packages when Docker is mapping users

- Arch's `makepkg` cannot be run as root

src/briefcase/platforms/linux/system.py

@@ -163,6 +163,21 @@ def finalize_app_config(self, app: AppConfig):
 
         if not self.use_docker:
             app.target_image = f"{app.target_vendor}:{app.target_codename}"
+        else:
+            if app.target_vendor_base == ARCH and self.tools.docker.is_users_mapped:
+                raise BriefcaseCommandError(
+                    """\
+Briefcase cannot use this Docker installation to target Arch Linux since the
+tools to build packages for Arch cannot be run as root.
+
+However, the Docker available to Briefcase requires the use of the root user in
+containers to maintain accurate file permissions of the build artefacts.
+
+This most likely means you're using Docker Desktop or rootless Docker.
+
+Install Docker Engine and try again or run Briefcase on an Arch host system.
+"""
+                )
 
         # Merge target-specific configuration items into the app config This
         # means:

tests/platforms/linux/system/test_mixin__finalize_app_config.py

@@ -12,7 +12,7 @@
 
 
 def test_docker(create_command, first_app_config):
-    "An app can be finalized inside docker"
+    """An app can be finalized inside docker."""
     # Build the app on a specific target
     create_command.target_image = "somevendor:surprising"
     create_command.tools.docker = MagicMock()
@@ -41,7 +41,7 @@ def test_docker(create_command, first_app_config):
 
 
 def test_nodocker(create_command, first_app_config, tmp_path):
-    "An app can be finalized without docker"
+    """An app can be finalized without docker."""
     # Build the app without docker
     create_command.target_image = None
     create_command.target_glibc_version = MagicMock(return_value="2.42")
@@ -77,7 +77,7 @@ def test_nodocker(create_command, first_app_config, tmp_path):
 
 
 def test_nodocker_non_freedesktop(create_command, first_app_config, tmp_path):
-    "If the system isn't FreeDesktop compliant raise an error"
+    """If the system isn't FreeDesktop compliant raise an error."""
     # Build the app without docker
     create_command.target_image = None
     create_command.target_glibc_version = MagicMock(return_value="2.42")
@@ -100,6 +100,57 @@ def test_nodocker_non_freedesktop(create_command, first_app_config, tmp_path):
         create_command.finalize_app_config(first_app_config)
 
 
+def test_docker_arch_with_user_mapping(create_command, first_app_config, tmp_path):
+    """If Docker is mapping users and the host system is Arch, an error is raised."""
+    # Build the app on a specific target
+    create_command.target_image = "somearch:surprising"
+    create_command.tools.docker = MagicMock()
+    create_command.tools.docker.is_users_mapped = True
+    create_command.target_glibc_version = MagicMock(return_value="2.42")
+
+    # Mock a minimal response for an Arch /etc/os-release
+    create_command.tools.docker.check_output.return_value = "\n".join(
+        [
+            "ID=arch",
+            "VERSION_ID=20230625.0.160368",
+        ]
+    )
+
+    # Finalize the app config
+    with pytest.raises(
+        BriefcaseCommandError,
+        match="Briefcase cannot use this Docker installation",
+    ):
+        create_command.finalize_app_config(first_app_config)
+
+
+def test_docker_arch_without_user_mapping(create_command, first_app_config, tmp_path):
+    """If Docker is *not* mapping users and the host system is Arch, an error is not
+    raised."""
+    # Build the app on a specific target
+    create_command.target_image = "somearch:surprising"
+    create_command.tools.docker = MagicMock()
+    create_command.tools.docker.is_users_mapped = False
+    create_command.target_glibc_version = MagicMock(return_value="2.42")
+
+    # Mock a minimal response for an Arch /etc/os-release
+    create_command.tools.docker.check_output.return_value = "\n".join(
+        [
+            "ID=arch",
+            "VERSION_ID=20230625.0.160368",
+        ]
+    )
+
+    # Finalize the app config
+    create_command.finalize_app_config(first_app_config)
+
+    # The app's image, vendor and codename have been constructed from the target image
+    assert first_app_config.target_image == "somearch:surprising"
+    assert first_app_config.target_vendor == "arch"
+    assert first_app_config.target_codename == "20230625"
+    assert first_app_config.target_vendor_base == "arch"
+
+
 def test_properties(create_command, first_app_config):
     """The final app config is the result of merging target properties, plus other
     derived properties."""
@@ -414,7 +465,7 @@ def test_properties_no_version(create_command, first_app_config):
 
 
 def test_passive_mixin(first_app_config, tmp_path):
-    "An app using the PassiveMixin can be finalized"
+    """An app using the PassiveMixin can be finalized."""
     run_command = LinuxSystemRunCommand(
         logger=Log(),
         console=Console(),