feat: add GitHub Actions cache for Docker layer builds

Use GHA cache backend (cache-from/cache-to) so that unchanged
layers (especially the slow ARM pip install) are reused across runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -83,3 +83,5 @@ jobs:
           push: true
           platforms: linux/amd64,linux/arm64,linux/arm/v7
           tags: ${{ steps.tags.outputs.tags }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max