perf: add GitHub Actions cache for Docker build layers

Uses type=gha cache with buildx so cargo-chef dependency layers
persist across workflow runs on ephemeral GitHub runners.

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

Author: knutties

.github/workflows/docker-publish.yml

@@ -46,3 +46,5 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max