* Improve Docker GPU setup diagnostics
Add a Docker GPU preflight script for NVIDIA users. The script is
read-only by default, checks host NVIDIA drivers, Docker availability,
and container GPU passthrough, and prints actionable next steps.
Add explicit opt-in modes to print install commands, install NVIDIA
Container Toolkit on Ubuntu/Debian, and enable the NVIDIA Compose overlay
in .env after passthrough is verified.
Document common NVIDIA Docker failure modes, ignore generated .env
backups, and clarify that Cookbook can only detect GPUs exposed to the
Odysseus container.
* Clarify Docker GPU diagnostic limits
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