ci: bump runner capacity to 4 and add BuildKit cache for image builds

- act_runner capacity 2 → 4 (QNAP host has 6 cores, leave 2 for OS)
- release-image: switch to docker/build-push-action@v5 with GHA cache
  (separate scopes for app/migrator to avoid cross-invalidation)

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

.gitea/gitea_compose_qnap_all_in_one.md

@@ -99,7 +99,7 @@ services:
           level: info
         runner:
           file: /data/.runner
-          capacity: 2
+          capacity: 4
           timeout: 3h
           insecure: false
           fetch_timeout: 5s