perf(ci): implement Docker Buildx with registry layer caching

[Peleke]

Problem

GitHub Actions rebuilds Docker images from scratch every time, even when most layers are unchanged.

Solution

• Use docker/setup-buildx-action
• Use docker/build-push-action with cache-from/cache-to
• Cache layers in Artifact Registry

Expected Impact

70-90% faster builds by reusing cached layers from previous builds

Dependencies

Should be implemented AFTER issues #2 and #3 for maximum benefit

[Peleke]

Merged