build: NuGet cache mount + buildx registry cache

Mirror the cache infra added to nas-auth (commit a1ecba3) and
obsidian-mcp. Same pattern: --mount=type=cache for restore+publish
in Dockerfile, registry cache mode=max in workflow.

.gitea/workflows/build-image.yml

@@ -66,6 +66,10 @@ jobs:
         with:
           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。
+          cache-from: type=registry,ref=git.zhengchentao.win/zhengchen.tao/gitea-mcp:buildcache
+          cache-to: type=registry,ref=git.zhengchentao.win/zhengchen.tao/gitea-mcp:buildcache,mode=max
           labels: |
             org.opencontainers.image.source=https://git.zhengchentao.win/zhengchen.tao/gitea-mcp
             org.opencontainers.image.revision=${{ steps.meta.outputs.full_sha }}

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.6
 # ── Stage 1: build ──────────────────────────────────────────────
 FROM mcr.microsoft.com/dotnet/sdk:10.0 AS builder
 
@@ -5,11 +6,13 @@ WORKDIR /src
 
 # 先复制 csproj，单独 restore（利用层缓存）
 COPY gitea-mcp.csproj .
-RUN dotnet restore gitea-mcp.csproj
+RUN --mount=type=cache,target=/root/.nuget/packages \
+    dotnet restore gitea-mcp.csproj
 
 # 复制剩余源码并发布
 COPY . .
-RUN dotnet publish gitea-mcp.csproj \
+RUN --mount=type=cache,target=/root/.nuget/packages \
+    dotnet publish gitea-mcp.csproj \
     -c Release \
     -o /app/publish \
     --no-restore