Source Code

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

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

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

# 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

# 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.

# 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

# 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:

// /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

# 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

# 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

# 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:

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:

MSYS_NO_PATHCONV=1 docker sandbox exec my-sandbox ls /home/agent

Sandbox won't start after Docker update

docker sandbox reset  # Clears all sandbox state

docker-sandbox