feat: add Docker daemon health check before image pulls

[boffin-dmytro]

Summary

• Add Docker daemon health verification before image pulls in phase 08
• Fail fast with clear error messages if daemon is not responding
• Prevents wasting time on lengthy pull phase when daemon is down

Rationale

Image pulls are the longest phase (~5-10 minutes). If the Docker daemon is not running, pulls will fail with cryptic errors. This change verifies daemon health upfront and provides actionable error messages.

Changes

• Check docker info before building image pull list
• Provide platform-specific troubleshooting hints (systemd, WSL2)
• Skip check in dry-run mode

Test Plan

• Verify check passes when daemon is running
• Verify check fails gracefully when daemon is stopped
• Verify error messages are actionable
• Verify dry-run mode skips the check

[boffin-dmytro]

Closing - will recreate against upstream repo