ci: add registry-based layer caching for Docker builds

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -90,10 +90,12 @@ jobs:
         run: |
           REGISTRY="gitea.jeanlucmakiola.de"
           IMAGE="${REGISTRY}/${{ gitea.repository_owner }}/gearbox"
-          docker buildx build -t "${IMAGE}:${VERSION}" -t "${IMAGE}:latest" .
           echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "$REGISTRY" -u "${{ gitea.repository_owner }}" --password-stdin
-          docker push "${IMAGE}:${VERSION}"
-          docker push "${IMAGE}:latest"
+          docker buildx build \
+            --cache-from type=registry,ref=${IMAGE}:buildcache \
+            --cache-to type=registry,ref=${IMAGE}:buildcache \
+            -t "${IMAGE}:${VERSION}" -t "${IMAGE}:latest" \
+            --push .
 
       - name: Create Gitea release
         run: |