ci: switch build cache from type=gha to type=registry

The Gitea Actions cache HTTP backend times out on export
(dial tcp 172.20.8.x:xxxxx: i/o timeout), failing the job even though
the image push itself succeeds.

Registry cache stores layer cache as a dedicated tag ':buildcache' in
the same container registry that already holds the images — reliable
and self-contained.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -57,5 +57,7 @@ jobs:
           platforms: linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          # Registry-based cache: uses the image registry itself for layer reuse.
+          # Avoids the unreliable Gitea Actions cache HTTP backend.
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
+          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache,mode=max