choredocker): enhance local Docker socket access and error handling

- Added guidance for Docker socket group access in docker-compose files.
- Introduced docker-compose.override.example.yml for supplemental group configuration.
- Improved entrypoint diagnostics to include socket GID and group guidance.
- Updated README with instructions for setting up Docker socket access.
- Enhanced backend error handling to provide actionable messages for permission issues.
- Updated frontend components to display troubleshooting information regarding Docker socket access.
- Added tests to ensure proper error messages and guidance are rendered in UI.
- Revised code coverage settings to include Docker service files for better regression tracking.

frontend/src/components/ProxyHostForm.tsx

@@ -651,7 +651,11 @@ export default function ProxyHostForm({ host, onSubmit, onCancel }: ProxyHostFor
                       </p>
                       <p className="text-gray-400">
                         <strong>Troubleshooting:</strong> Ensure Docker is running and the socket is accessible.
-                        If running in a container, mount <code className="text-xs bg-gray-800 px-1 py-0.5 rounded">/var/run/docker.sock</code>.
+                        If running in a container, mount <code className="text-xs bg-gray-800 px-1 py-0.5 rounded">/var/run/docker.sock</code> and
+                        ensure the container has access to the Docker socket group
+                        (e.g., <code className="text-xs bg-gray-800 px-1 py-0.5 rounded">group_add</code> in
+                        Compose or <code className="text-xs bg-gray-800 px-1 py-0.5 rounded">--group-add</code> with
+                        Docker&nbsp;CLI).
                       </p>
                     </div>
                   </div>