Docker-python fix for get_ubuntu_version() (#4430)

ultralytics/engine/exporter.py

@@ -424,7 +424,9 @@ class Exporter:
                 'https://github.com/pnnx/pnnx/.\nNote PNNX Binary file must be placed in current working directory '
                 f'or in {ROOT}. See PNNX repo for full installation instructions.')
             _, assets = get_github_assets(repo='pnnx/pnnx', retry=True)
-            asset = [x for x in assets if ('macos' if MACOS else 'ubuntu' if LINUX else 'windows') in x][0]
+            system = 'macos' if MACOS else 'ubuntu' if LINUX else 'windows'  # operating system
+            asset = [x for x in assets if system in x][0] if assets else \
+                f'https://github.com/pnnx/pnnx/releases/download/20230816/pnnx-20230816-{system}.zip'  # fallback
             attempt_download_asset(asset, repo='pnnx/pnnx', release='latest')
             unzip_dir = Path(asset).with_suffix('')
             pnnx = ROOT / pnnx_filename  # new location