Fix model wrapper logger, and better XP around chains docker image va…

[marius-baseten]

• [truss] Use uvicorn logger for model load, it's correctly configured and doesn't get messed up by numpy (c.f. BUG: importing numpy.distutils replaces the global logger with a custom class which doesn't support kwargs numpy/numpy#24213).
• [truss] Add test to assert model-load-thread logs are present (they might get easily lost if the logging isn't setup in a particular way).
• [chains] Display a clear error message if base_image is specified as string and link to docs.

🚀 What

💻 How

🔬 Testing