The diffusers video path uses SageAttention (INT8 attention) when available
for faster Wan2.2 rendering. Like flash-attn it is CUDA-arch-sensitive, so
it is built from source in the devel/builder stage against the just-installed
torch, gated by BUILD_SAGEATTENTION (default 1) and SAGEATTENTION_ARCH
(default 8.6 = RTX 3090). The build is non-fatal: on failure the image still
works and the runtime attention-backend resolver falls back to flash/SDPA.

build_oci_image.sh passes the three new args through (overridable via env:
BUILD_SAGEATTENTION / SAGEATTENTION_REF / SAGEATTENTION_ARCH).

Note: the fast update_oci_image.sh overlay is based on the runtime image (no
nvcc), so it cannot build SageAttention — a full build_oci_image.sh is needed
to bake it in.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce