feat(sandbox): replace one-shot docker run with session-scoped containers

docker/Dockerfile.sandbox

@@ -23,9 +23,13 @@ COPY --from=node-base /usr/local/lib/node_modules /usr/local/lib/node_modules
 RUN ln -s /usr/local/lib/node_modules/npm/bin/npm-cli.js /usr/local/bin/npm && \
     ln -s /usr/local/lib/node_modules/npm/bin/npx-cli.js /usr/local/bin/npx
 
-# Install minimal CLI tools (bash/grep/sed/coreutils already in slim image)
+# Install minimal CLI tools (bash/grep/sed/coreutils/findutils already in slim
+# image). curl is not optional: the sandbox connectivity check probes egress
+# with it, and skills that fetch a URL expect it to be there.
 RUN apt-get update && apt-get install -y --no-install-recommends \
     jq \
+    curl \
+    ca-certificates \
     && rm -rf /var/lib/apt/lists/* /var/cache/apt/*
 
 # Note: Current preloaded skills only use Python stdlib
@@ -39,6 +43,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN groupadd -g 1000 user && \
     useradd -u 1000 -g user -m -s /bin/bash user
 
+# Pre-create the two directories every backend writes to and hand them to the
+# sandbox account. WeKnora otherwise has to create and chown them as root on
+# each execution, which needs capabilities a hardened container should not
+# have to keep.
+RUN mkdir -p /workspace/input /workspace/output && \
+    chown -R user:user /workspace
+
 WORKDIR /workspace
 USER user
 

docs/agent-skills.md

@@ -132,7 +132,7 @@ Docker、Local、CubeSandbox、E2B 均通过同一套空间配置 CRUD、连接
 
 | 模式 | 状态 | 说明 |
 |------|------|------|
-| `docker` | 稳定 | 每次执行启动短生命周期容器；镜像和环境变量按空间配置，不保留会话绑定 |
+| `docker` | 稳定 | 单机 Docker daemon；会话级持久（一个会话一个长驻容器），支持多机 WeKnora 副本（需 Redis），但沙箱都落在同一台 daemon 上。见 [Docker 沙箱后端](sandbox-docker-backend.md) |
 | `local` | 开发 | 直接在 WeKnora 服务主机执行；无容器/MicroVM 隔离，不保留会话绑定 |
 | `cube` | 稳定 | Tencent CubeSandbox MicroVM；会话级持久，支持多机（需 Redis） |
 | `e2b` | 稳定 | E2B 云端 MicroVM；会话级持久，支持多机（需 Redis）；依赖第三方 SDK go-e2b |
@@ -538,7 +538,7 @@ docker pull wechatopenai/weknora-sandbox:latest
 sh scripts/build_images.sh -s
 ```
 
-> 如果未预拉取，应用启动时会自动异步拉取镜像（`EnsureImage`），但首次执行可能需要等待下载完成。
+> 如果未预拉取，创建第一个沙箱时会先拉取镜像，首次执行需要等待下载完成；也可以在设置页的模板步骤提前触发拉取。
 
 **镜像内置环境**：
 - Python 3.11 + pip（requests、pyyaml、pandas、beautifulsoup4）

docs/poc/docker-sandbox/README.md

@@ -0,0 +1,32 @@
+# Docker 沙箱后端可行性 PoC
+
+配套文档：[Docker 沙箱后端重做调研](../../sandbox-docker-backend.md)。
+
+这个程序直接打 Docker Engine API，逐项验证「Docker 能不能承载 WeKnora
+`RemoteSandboxClient` 契约 + E2B 那套 Snapshot 工作流」。它不是产品代码，也不参与主模块构建
+（自带 `go.mod`），只作为调研结论的可复现证据。
+
+每一项都打印 `PASS/FAIL` 和实际观测值。带 `(GAP)` 的步骤断言的是「Docker 做不到什么」，
+它们同样以 PASS 结束——PASS 表示差距被复现了，不是表示能力具备。
+
+## 跑法
+
+需要一个可达的 Docker daemon（尊重 `DOCKER_HOST`），能拉 `python:3.11-slim`：
+
+```bash
+cd docs/poc/docker-sandbox
+go run .            # daemon 在本机 unix socket 上时可能需要 sudo -E
+```
+
+程序会自建一个带 uid 1000 `user` 账号的模板镜像（对齐 E2B 模板约定），跑完自行删除它创建的容器。
+留下的 `weknora-poc/*` 镜像用 `docker image rm` 清理。
+
+## 覆盖的内容
+
+- 生命周期：Create / Connect（换客户端重连）/ List（label 过滤）/ Delete / Pause / Stop+Start
+- 执行：user、workdir、env、stdin、stdout+stderr 分离、退出码、超时
+- 文件面：archive API 读写与 stat、exec 实现的 mkdir/ls/rm
+- 会话语义：`pip install` 与写文件跨 exec 存活
+- Snapshot：commit 成镜像、从快照启新沙箱、v1→v2 增量、按 label 列举、层数与体积
+- 差距复现：客户端取消不杀进程、CapDrop ALL 后 root 越不过权限位、快照不含内存态、
+  daemon 无空闲 TTL、kill `docker run` 会留下还在跑的容器

docs/poc/docker-sandbox/go.mod

@@ -0,0 +1,29 @@
+module github.com/Tencent/WeKnora/docs/poc/docker-sandbox
+
+go 1.26.0
+
+require (
+	github.com/moby/moby/api v1.55.0
+	github.com/moby/moby/client v0.5.1
+)
+
+require (
+	github.com/Microsoft/go-winio v0.6.2 // indirect
+	github.com/containerd/errdefs v1.0.0 // indirect
+	github.com/containerd/errdefs/pkg v0.3.0 // indirect
+	github.com/distribution/reference v0.6.0 // indirect
+	github.com/docker/go-connections v0.7.0 // indirect
+	github.com/docker/go-units v0.5.0 // indirect
+	github.com/felixge/httpsnoop v1.0.4 // indirect
+	github.com/go-logr/logr v1.4.2 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/moby/docker-image-spec v1.3.1 // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
+	github.com/opencontainers/image-spec v1.1.1 // indirect
+	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.60.0 // indirect
+	go.opentelemetry.io/otel v1.35.0 // indirect
+	go.opentelemetry.io/otel/metric v1.35.0 // indirect
+	go.opentelemetry.io/otel/trace v1.35.0 // indirect
+	golang.org/x/sys v0.33.0 // indirect
+)

docs/poc/docker-sandbox/go.sum

@@ -0,0 +1,61 @@
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