feat: Add ART GPU image build script

[bradhilton]

Summary

• Add an ART-specific Kubernetes BuildKit script for building, pushing, and prewarming the GPU image.
• Add an ART GPU Dockerfile that warms backend dependencies and bakes the SkyPilot runtime.
• Point ART SkyPilot configs at the new image and update Kubernetes pod settings/env handling needed for fast launches.

Test plan

• bash -n scripts/build-gpu-image.sh scripts/setup.sh
• git diff --check
• uv lock --check
• Built and pushed docker.io/bradhiltonnw/art-gpu:latest@sha256:284c62debc995d5cec96a595820581ffb28b6ee464e64575da62c19760f16b4f
• Prewarmed 10/10 GPU nodes with the pushed digest
• Ran ART SkyPilot smoke launch: ART image smoke ok 2.11.0+cu128, total 1:04.23