fix(docker): add User-Agent header to model downloads

The Berkeley colorization server returns 403 for the default
Python-urllib User-Agent. Setting a global opener fixes this
for all model downloads.

docker/download_models.py

@@ -7,6 +7,11 @@ import os
 import sys
 import urllib.request
 
+# Some servers (e.g. Berkeley) block the default Python-urllib User-Agent.
+_opener = urllib.request.build_opener()
+_opener.addheaders = [("User-Agent", "Stirling-Image/1.0")]
+urllib.request.install_opener(_opener)
+
 # Force CPU mode during build - no GPU driver available at build time.
 # Must be set before any ML library import.
 os.environ["PADDLE_DEVICE"] = "cpu"