ci: cache-to use mode=min + ignore-error to unblock deploy

Same fix as nas-auth: mode=max takes ~4min and times out on the NAS,
which marks the build as failure and skips deploy. mode=min is fast
and ignore-error keeps the build from failing on cache export.

.gitea/workflows/build-image.yml

@@ -71,9 +71,13 @@ jobs:
           context: .
           push: true
           # buildx 每次 CI 起新的 buildkit 实例，layer cache 默认会丢。用 registry
-          # 当跨 CI run 的缓存。mode=max 把所有中间层（含 NuGet restore 那层）都存。
+          # 当跨 CI run 的缓存。
+          # mode=min：只推最终镜像层引用（默认）。mode=max 会推所有中间层（实测加密
+          # 机械盘 + Gitea registry 每次 ~4min 还超时，得不偿失）。
+          # ignore-error=true：万一 cache export 出问题不让 build 标 failure，
+          # 否则 deploy job 会被 skip。
           cache-from: type=registry,ref=git.zhengchentao.win/zhengchen.tao/obsidian-mcp:buildcache
-          cache-to: type=registry,ref=git.zhengchentao.win/zhengchen.tao/obsidian-mcp:buildcache,mode=max
+          cache-to: type=registry,ref=git.zhengchentao.win/zhengchen.tao/obsidian-mcp:buildcache,mode=min,ignore-error=true
           labels: |
             org.opencontainers.image.source=https://git.zhengchentao.win/zhengchen.tao/obsidian-mcp
             org.opencontainers.image.revision=${{ steps.meta.outputs.full_sha }}