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Duet can be deployed using Docker for easy setup and portability. The Docker image includes all necessary dependencies for pair programming sessions.

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Dockerfile Overview

The Duet Dockerfile uses a multi-stage build:
  1. Build stage - Compiles the Go binary
  2. Run stage - Creates a minimal runtime environment with development tools

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What’s Included

The Docker image includes:
  • Neovim - Text editor for collaborative coding
  • Node.js & npm - JavaScript runtime and package manager
  • OpenSSH - SSH server and key management
  • Go binary - Compiled Duet server

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Quick Start

1

Build the Docker image

docker build -t duet .
2

Run the container

docker run -p 2222:2222 duet
3

Connect via SSH

ssh <username>@localhost -p 2222

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Configuration

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Environment Variables

The Docker container accepts configuration through command-line arguments: 
docker run -p 2222:2222 duet /app/duet \
  -addr :2222 \
  -hostkey /app/.ssh/id_ed25519 \
  -worker https://your-worker.workers.dev

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Port Mapping

The Dockerfile exposes port 2222 by default: 
EXPOSE 2222
Map it to a different host port: 
docker run -p 22:2222 duet  # Maps host port 22 to container port 2222

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Volume Mounts

Persist workspaces across container restarts: 
docker run -p 2222:2222 \
  -v $(pwd)/workspaces:/app/workspaces \
  duet
Each room gets its own subdirectory under /app/workspaces.

​
Custom Worker URL

To use your own Cloudflare Worker: 
docker run -p 2222:2222 duet \
  /app/duet -addr :2222 \
  -hostkey /app/.ssh/id_ed25519 \
  -worker https://duet-cf-worker.<subdomain>.workers.dev

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Docker Compose

Create a docker-compose.yml file: 
version: '3.8'

services:
  duet:
    build: .
    ports:
      - "2222:2222"
    volumes:
      - ./workspaces:/app/workspaces
    command: [
      "/app/duet",
      "-addr", ":2222",
      "-hostkey", "/app/.ssh/id_ed25519",
      "-worker", "https://your-worker.workers.dev"
    ]
    restart: unless-stopped
Run with: 
docker-compose up -d

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Production Deployment

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Fly.io

Duet includes a fly.toml configuration for deployment on Fly.io: 
app = 'duet-morning-meadow-5890'
primary_region = 'lhr'

[build]

[http_service]
  internal_port = 2222
  force_https = true
  auto_stop_machines = 'stop'
  auto_start_machines = true
  min_machines_running = 0
  processes = ['app']

[[services]]
  protocol = 'tcp'
  internal_port = 2222

  [[services.ports]]
    port = 22

[[vm]]
  memory = '1gb'
  cpu_kind = 'shared'
  cpus = 1
1

Install Fly CLI

curl -L https://fly.io/install.sh | sh
2

Login to Fly.io

fly auth login
3

Deploy the application

fly deploy
4

Set secrets

If you need to configure the Worker URL via secrets:
fly secrets set WORKER_URL=https://your-worker.workers.dev

​
Resource Configuration

The default Fly.io configuration uses:
  • Memory: 1GB
  • CPU: 1 shared CPU
  • Auto-scaling: Machines stop when idle, start on demand
  • Minimum running: 0 (cost-effective for low traffic)
The auto_stop_machines and auto_start_machines settings help reduce costs by stopping idle machines.

​
Security Considerations

​
SSH Host Keys

The Dockerfile generates SSH host keys at build time: 
RUN mkdir -p /app/.ssh && \
    ssh-keygen -t ed25519 -f /app/.ssh/id_ed25519 -N "" && \
    chown -R duet:duet /app/.ssh
For production, consider mounting a persistent host key to avoid key changes on container restarts:
docker run -p 2222:2222 \
  -v $(pwd)/ssh-keys:/app/.ssh \
  duet

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Non-Root User

The container runs as a non-root user duet for security: 
RUN adduser -D duet
USER duet

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Troubleshooting

​
Check Container Logs

docker logs <container-id>

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Interactive Shell

Access the container: 
docker exec -it <container-id> /bin/sh

​
Verify SSH Key

Check if the host key exists: 
docker exec <container-id> ls -la /app/.ssh

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Common Issues

  • Connection refused: Verify port mapping is correct
  • Permission denied: Check SSH host key permissions
  • Worker not responding: Verify Worker URL is accessible from container

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