Test Docker images in CI

[aktech]

Summary

CI currently deploys with pre-built images from the registry. We should also build and test images locally in CI to catch Dockerfile issues before merging.

Context

This was skipped due to disk space limitations on GitHub runners. The local image builds + k3d import exceeded available space.

Note: Local development already has hot reload via Tilt (docker_build in Tiltfile), so this is specifically for catching issues in PRs.

Potential solutions

1. Use easimon/maximize-build-space action to free ~30GB
2. Use larger GitHub runners (requires org config)
3. Use registry cache from build-image workflow instead of GHA cache
4. Use cirun.io for cloud runners with more resources

Acceptance criteria

• CI builds images locally from images/Dockerfile
• Images are imported to k3d cluster
• Helm deploys with locally built images
• Tests pass with locally built images