docker-sandbox-setup

Configure Docker containers to run Claude Code in isolated, safe environments that prevent AI agents from modifying the host system.

Purpose

This skill provides Docker configuration knowledge specifically for creating sandboxed development environments. Focus on the minimum Docker setup needed for Claude Code isolation while maintaining full development capabilities, not general Docker expertise.

Core Concepts

Isolation Strategy

Sandboxes use Docker to create complete isolation between Claude Code and the host system:

• Container boundary: All Claude Code operations occur within the container
• Volume mounts: Workspace shared between host and container for file access
• Network isolation: Port forwarding for web development, no direct host network access
• Persistent caches: Package managers and build caches preserved across restarts

Base Image Selection

Default recommendation: Ubuntu 24.04 LTS

FROM ubuntu:24.04

Why Ubuntu 24.04:

• Stable LTS release with long support window
• Widely used, extensive package availability
• Good balance of stability and modern tooling

Alternative images:

• debian:bookworm - Minimal, smaller image size
• ubuntu:25.04 - Latest features, shorter support

Custom Base Images

For faster sandbox creation with common tools pre-installed, build a custom base image:

When to use custom base images:

• Creating multiple sandboxes with similar tooling
• Want faster sandbox initialization
• Standard tool set across all projects

How to create:

# my-sandbox-base.dockerfile
FROM ubuntu:24.04

# Install common tools
RUN apt-get update && apt-get install -y \
    curl wget git build-essential \
    ripgrep fd-find jq python3 python3-pip \
    && rm -rf /var/lib/apt/lists/*

# Install Rust
RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
ENV PATH="/root/.cargo/bin:${PATH}"

# Install Node.js via nvm
RUN curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.1/install.sh | bash

Build and tag:

docker build -f my-sandbox-base.dockerfile -t my-sandbox-base:latest .

Use in Sandbox.toml:

[sandbox]
base_image = "my-sandbox-base:latest"

Container Configuration

Volume Mounts

Mount the workspace and persistence directories:

docker run \
  -v $(pwd)/workspace:/workspace \
  -v $(pwd)/.docker-cache/cargo:/root/.cargo \
  -v $(pwd)/.docker-cache/npm:/root/.npm \
  -v $(pwd)/.docker-cache/pip:/root/.cache/pip \
  -v $(pwd)/.docker-cache/claude:/root/.claude \
  sandbox-container

Critical mounts:

• /workspace - Project source code (editable from host)
• /root/.cargo - Rust package cache
• /root/.npm - Node.js package cache
• /root/.cache/pip - Python package cache
• /root/.claude - Claude Code config and cache

Benefits:

• Source code editable from host
• Packages persist across container restarts
• Claude Code authentication persists
• Fast container rebuilds

Port Forwarding

Forward ports for web development:

docker run -p 3000-3999:3000-3999 sandbox-container

Port range rationale:

• Common dev server ports (3000, 3001, etc.)
• Supports multiple services in monorepos
• Wide enough for flexibility, narrow enough for security

Custom ports: Detect from package.json scripts or user specification:

# Specific ports only
docker run -p 3000:3000 -p 8080:8080 sandbox-container

Environment Variables

Pass through essential environment variables:

docker run \
  --env-file .env \
  -e GITHUB_TOKEN="${GITHUB_TOKEN}" \
  sandbox-container

Always pass through:

• GITHUB_TOKEN - For gh CLI operations
• Custom API keys as specified in .env

Never pass through:

• Sensitive host environment variables
• System paths or credentials

Dockerfile Structure

Basic Template

FROM ubuntu:24.04

# Set non-interactive frontend for apt
ENV DEBIAN_FRONTEND=noninteractive

# Install base packages
RUN apt-get update && apt-get install -y \
    curl wget git build-essential \
    ca-certificates gnupg \
    && rm -rf /var/lib/apt/lists/*

# Set working directory
WORKDIR /workspace

# Install languages (Rust, Python, Node.js)
# ... language-specific setup ...

# Install CLI tools
# ... tool installation ...

# Install Claude Code
RUN curl -fsSL https://claude.ai/install.sh | bash

# Configure shell
# ... shell setup ...

CMD ["/bin/bash"]

Layer Optimization

Order Dockerfile commands by change frequency:

1. Base packages (rarely change)
2. Language runtimes (occasionally update)
3. CLI tools (occasionally update)
4. Project dependencies (change frequently)
5. Source code (changes constantly - use volume mount instead)

Good ordering:

# Rarely changes - cached well
FROM ubuntu:24.04
RUN apt-get update && apt-get install -y curl git

# Occasionally changes - medium cache hit rate
RUN install_rust_stable

# Frequently changes - volume mounted, not in image
# (Don't COPY source code into image)

Why this matters:

• Earlier layers cached longer
• Changes to later layers don't invalidate earlier cache
• Faster rebuilds during development

Multi-Stage Considerations

Multi-stage builds are typically NOT needed for sandbox containers because:

• Containers are development environments, not production images
• Need full tooling available for development
• Size optimization less critical than functionality

Skip multi-stage builds for sandboxes.

Container Lifecycle

Starting a Sandbox

#!/bin/bash
# up.sh

docker build -t sandbox-${PROJECT_NAME} .
docker run -d \
  --name sandbox-${PROJECT_NAME} \
  -v $(pwd)/workspace:/workspace \
  -v $(pwd)/.docker-cache/cargo:/root/.cargo \
  -v $(pwd)/.docker-cache/npm:/root/.npm \
  -v $(pwd)/.docker-cache/pip:/root/.cache/pip \
  -v $(pwd)/.docker-cache/claude:/root/.claude \
  -p 3000-3999:3000-3999 \
  --env-file .env \
  sandbox-${PROJECT_NAME}

echo "Sandbox is starting..."

# Wait and check if the container is running
sleep 2
if docker ps | grep -q "sandbox-${PROJECT_NAME}"; then
    echo "✅ Sandbox is running!"
    echo "Access it with: ./sandbox/shell.sh"
else
    echo "❌ Failed to start sandbox."
    echo "Check logs: docker logs sandbox-${PROJECT_NAME}"
    exit 1
fi

Interactive Shell

#!/bin/bash
# shell.sh

docker exec -it sandbox-${PROJECT_NAME} /bin/zsh

Running Commands

#!/bin/bash
# run.sh

docker exec sandbox-${PROJECT_NAME} "$@"

Stopping a Sandbox

#!/bin/bash
# stop.sh

docker stop sandbox-${PROJECT_NAME}
docker rm sandbox-${PROJECT_NAME}

Common Patterns

Detecting Container Environment

Add visual indicators that you're in a container:

# In .zshrc
if [ -f /.dockerenv ]; then
    export IN_CONTAINER=true
    # Starship will show container indicator
fi

Starship config recognizes this:

[container]
disabled = false
symbol = "🐳 "
style = "bold red"

Preserving Build Context

Use .dockerignore to exclude unnecessary files:

# .dockerignore
.git
node_modules/
.docker-cache/
*.log
.env.local

Benefits:

• Faster builds (smaller context)
• Prevents accidentally copying sensitive files
• Reduces image size

Health Checks

Add health checks for long-running services:

HEALTHCHECK --interval=30s --timeout=3s \
  CMD curl -f http://localhost:3000/ || exit 1

Use sparingly - only for services that need it.

Security Considerations

File Permissions

Container runs as root by default. Files created in workspace will be root-owned.

Solution 1: Match host UID/GID

ARG USER_ID=1000
ARG GROUP_ID=1000

RUN groupadd -g ${GROUP_ID} developer && \
    useradd -u ${USER_ID} -g developer developer

USER developer

Solution 2: Fix permissions on stop

# In stop.sh
docker exec sandbox-${PROJECT_NAME} chown -R $(id -u):$(id -g) /workspace
docker stop sandbox-${PROJECT_NAME}

Choose Solution 2 for simplicity.

Network Isolation

Containers are network-isolated by default. Only exposed ports are accessible.

Don't expose:

• Port 22 (SSH)
• Database ports unless required
• Internal service ports

Secret Management

Never build secrets into the image:

# ❌ BAD
RUN git clone https://user:password@github.com/repo.git

# ✅ GOOD
RUN git clone https://github.com/repo.git
# (Requires GITHUB_TOKEN in environment)

Use .env files and --env-file flag instead.

Additional Resources

Reference Files

For detailed troubleshooting and optimization:

• references/troubleshooting.md - Common Docker issues and solutions
• references/optimization.md - Performance and DevX improvements

Example Files

Complete working examples:

• examples/Dockerfile.template - Full Dockerfile template with all features
• examples/docker-compose.yml - Alternative using Docker Compose

Integration with Other Skills

With language-environment-config:

• This skill handles Docker container setup
• language-environment-config handles language installation inside container
• Use both together when generating Dockerfiles

With sandbox-config-management:

• This skill implements what Sandbox.toml specifies
• Read base_image, ports, volumes from Sandbox.toml
• Generate Docker configuration matching the config file

Quick Reference

Default base image: ubuntu:24.04

Required volume mounts:

• /workspace - Source code
• /root/.cargo - Rust cache
• /root/.npm - Node cache
• /root/.cache/pip - Python cache
• /root/.claude - Claude Code config

Common port range: -p 3000-3999:3000-3999

Environment: --env-file .env + -e GITHUB_TOKEN

Order Dockerfile: Base → Languages → Tools → Config

Skip multi-stage builds for dev containers