Docker training mode commented out — restore as optional flag

[2imi9]

Problem

V4 runner has Docker training commented out. Runs locally with FlexAttention.

Fix

Add env var toggle: AUTORESEARCH_DOCKER=1 for Docker mode.
Docker needed when FA4 lands (Linux-only CUTLASS DSL).

Docker Image Fixes

• Capability-based FA3 repo selection
• Add cache volume mount
• Add FA4 when available