[Feature] Start Sandbox From Dockerfile

**Feature Category**
- [x] Sandbox
- [ ] Actions
- [ ] Deployments
- [ ] SDK & API
- [ ] Envhub
- [ ] CLI
- [ ] Performance & Optimization
- [ ] Documentation & Examples

**Problem Statement**
当前 ROCK Sandbox 仅支持从预构建的 Docker 镜像（如 python:3.11）启动。期望能从 Dockerfile 或包含 Dockerfile 的目录直接启动 Sandbox。

**Proposed Solution**

**Detailed Feature Description**
case 1: 从单一Dockerfile启动sandbox
case 2: 从包含Dockerfile的目录中启动sandbox，Dockerfile依赖本地的文件
用例详见附录的文件

[cases.zip](https://github.com/user-attachments/files/25619483/cases.zip)

Start by inspecting the sandbox startup entry points and the attached cases.zip, which describes launching from a Dockerfile and from a directory with local dependencies. Trace the existing prebuilt-image flow and determine the build-context behavior needed for both cases; done means both documented cases can start a sandbox successfully.

Written by the indexing model from the issue text.