Docker Sandbox Lucas

# Docker Sandbox

Run agents and commands in isolated VM environments using Docker Desktop's sandbox feature. Each sandbox gets its own lightweight VM with filesystem isolation, network proxy controls, and workspace mounting via virtiofs.

When to Use

• Exploring untrusted packages or skills before installing them system-wide
• Running arbitrary code from external sources safely
• Testing destructive operations without risking the host
• Isolating agent workloads that need network access controls
• Setting up reproducible environments for experiments

Requirements

• Docker Desktop 4.49+ with the `docker sandbox` plugin
• Verify: `docker sandbox version`

Quick Start

Create a sandbox for the current project

```bash docker sandbox create --name my-sandbox claude . ```

• This creates a VM-isolated sandbox with:
• The current directory mounted via virtiofs
• Node.js, git, and standard dev tools pre-installed
• Network proxy with allowlist controls

Run commands inside

```bash docker sandbox exec my-sandbox node --version docker sandbox exec my-sandbox npm install -g some-package docker sandbox exec -w /path/to/workspace my-sandbox bash -c "ls -la" ```

Run an agent directly

```bash # Create and run in one step docker sandbox run claude . -- -p "What files are in this project?"

# Run with agent arguments after -- docker sandbox run my-sandbox -- -p "Analyze this codebase" ```

Commands Reference

Lifecycle

```bash # Create a sandbox (agents: claude, codex, copilot, gemini, kiro, cagent) docker sandbox create --name <name> <agent> <workspace-path>

# Run an agent in sandbox (creates if needed) docker sandbox run <agent> <workspace> [-- <agent-args>...] docker sandbox run <existing-sandbox> [-- <agent-args>...]

# Execute a command docker sandbox exec [options] <sandbox> <command> [args...] -e KEY=VAL # Set environment variable -w /path # Set working directory -d # Detach (background) -i # Interactive (keep stdin open) -t # Allocate pseudo-TTY

# Stop without removing docker sandbox stop <sandbox>

# Remove (destroys VM) docker sandbox rm <sandbox>

# List all sandboxes docker sandbox ls

# Reset all sandboxes docker sandbox reset

# Save snapshot as reusable template docker sandbox save <sandbox> ```

Network Controls

The sandbox includes a network proxy for controlling outbound access.

```bash # Allow specific domains docker sandbox network proxy <sandbox> --allow-host example.com docker sandbox network proxy <sandbox> --allow-host api.github.com

# Block specific domains docker sandbox network proxy <sandbox> --block-host malicious.com

# Block IP ranges docker sandbox network proxy <sandbox> --block-cidr 10.0.0.0/8

# Bypass proxy for specific hosts (direct connection) docker sandbox network proxy <sandbox> --bypass-host localhost

# Set default policy (allow or deny all by default) docker sandbox network proxy <sandbox> --policy deny # Block everything, then allowlist docker sandbox network proxy <sandbox> --policy allow # Allow everything, then blocklist

# View network activity docker sandbox network log <sandbox> ```

Custom Templates

```bash # Use a custom container image as base docker sandbox create --template my-custom-image:latest claude .

# Save current sandbox state as template for reuse docker sandbox save my-sandbox ```

Workspace Mounting

The workspace path on the host is mounted into the sandbox via virtiofs. The mount path inside the sandbox preserves the host path structure:

| Host OS | Host Path | Sandbox Path | |---|---|---| | Windows | `H:\Projects\my-app` | `/h/Projects/my-app` | | macOS | `/Users/me/projects/my-app` | `/Users/me/projects/my-app` | | Linux | `/home/me/projects/my-app` | `/home/me/projects/my-app` |

The agent's home directory is `/home/agent/` with a symlinked `workspace/` directory.

Environment Inside the Sandbox

• Each sandbox VM includes:
• Node.js (v20.x LTS)
• Git (latest)
• Python (system)
• curl, wget, standard Linux utilities
• npm (global install directory at `/usr/local/share/npm-global/`)
• Docker socket (at `/run/docker.sock` - Docker-in-Docker capable)

Proxy Configuration (auto-set)

``` HTTP_PROXY=http://host.docker.internal:3128 HTTPS_PROXY=http://host.docker.internal:3128 NODE_EXTRA_CA_CERTS=/usr/local/share/ca-certificates/proxy-ca.crt SSL_CERT_FILE=/usr/local/share/ca-certificates/proxy-ca.crt ```

Important: Node.js `fetch` (undici) does NOT respect `HTTP_PROXY` env vars by default. For npm packages that use `fetch`, create a require hook:

```javascript // /tmp/proxy-fix.js const proxy = process.env.HTTPS_PROXY || process.env.HTTP_PROXY; if (proxy) { const { ProxyAgent } = require('undici'); const agent = new ProxyAgent(proxy); const origFetch = globalThis.fetch; globalThis.fetch = function(url, opts = {}) { return origFetch(url, { ...opts, dispatcher: agent }); }; } ```

Run with: `node -r /tmp/proxy-fix.js your-script.js`

Patterns

Safe Package Exploration

```bash # Create isolated sandbox docker sandbox create --name pkg-test claude .

# Restrict network to only npm registry docker sandbox network proxy pkg-test --policy deny docker sandbox network proxy pkg-test --allow-host registry.npmjs.org docker sandbox network proxy pkg-test --allow-host api.npmjs.org

# Install and inspect the package docker sandbox exec pkg-test npm install -g suspicious-package docker sandbox exec pkg-test bash -c "find /usr/local/share/npm-global/lib/node_modules/suspicious-package -name '*.js' | head -20"

# Check for post-install scripts, network calls, file access docker sandbox network log pkg-test

# Clean up docker sandbox rm pkg-test ```

Persistent Dev Environment

```bash # Create once docker sandbox create --name dev claude ~/projects/my-app

# Use across sessions docker sandbox exec dev npm test docker sandbox exec dev npm run build

# Save as template for team sharing docker sandbox save dev ```

Locked-Down Agent Execution

```bash # Deny-all network, allow only what's needed docker sandbox create --name secure claude . docker sandbox network proxy secure --policy deny docker sandbox network proxy secure --allow-host api.openai.com docker sandbox network proxy secure --allow-host github.com

# Run agent with restrictions docker sandbox run secure -- -p "Review this code for security issues" ```

Troubleshooting

"client version X is too old" Update Docker Desktop to 4.49+. The sandbox plugin requires engine API v1.44+.

"fetch failed" inside sandbox Node.js `fetch` doesn't use the proxy. Use the proxy-fix.js require hook above, or use `curl` instead: ```bash docker sandbox exec my-sandbox curl -sL https://api.example.com/data ```

Path conversion on Windows (Git Bash / MSYS2) Git Bash converts `/path` to `C:/Program Files/Git/path`. Prefix commands with: ```bash MSYS_NO_PATHCONV=1 docker sandbox exec my-sandbox ls /home/agent ```

Sandbox won't start after Docker update ```bash docker sandbox reset # Clears all sandbox state ```