devcontainer

name	devcontainer
description	Diagnose devcontainer configuration problems and guide development environment setup. This skill should be used when the user asks to 'set up devcontainer', 'fix container startup', 'configure VS Code dev container', 'Codespaces setup', or has Docker development environment issues. Keywords: devcontainer, docker, VS Code, Codespaces, container, development environment, Dockerfile.
license	MIT
compatibility	Requires Docker Desktop or compatible container runtime. Works with VS Code and GitHub Codespaces.
metadata	{"author":"jwynia","version":"1.0","type":"diagnostic","mode":"diagnostic+generative","domain":"development"}

Devcontainer Diagnostic

Diagnose devcontainer and Docker development environment problems. Help create reproducible, fast-starting development environments that work consistently across VS Code, GitHub Codespaces, and team members.

When to Use This Skill

Use this skill when:

• Setting up a new devcontainer
• Container startup is too slow
• Configuration errors or conflicts
• Different behavior in VS Code vs Codespaces
• Multi-service development environment needed

Do NOT use this skill when:

• Writing application code
• Deploying to production
• Configuring CI/CD pipelines

Core Principle

Development containers should provide instant productivity. Every configuration choice affects startup time, reproducibility, and team onboarding. Make these trade-offs explicit.

Diagnostic States

DV0: No Devcontainer Strategy

Symptoms: Manual setup, "check the README", works on one machine fails on others

Interventions:

• Start with pre-built devcontainer base image
• Use assets/devcontainer-simple.md template

DV1: Slow Container Startup

Symptoms: 5+ minute startup, heavy postCreateCommand, avoiding rebuilds

Key Questions:

• How long does startup actually take?
• What's in postCreateCommand?
• Are you using prebuilds?

Interventions:

• Move npm install/pip install to Dockerfile (cached)
• Use mcr.microsoft.com/devcontainers/* base images
• Configure prebuilds for team repos
• Run scripts/analyze-devcontainer.ts

DV2: Configuration Problems

Symptoms: JSON errors, VS Code won't connect, features conflicting

Checklist:

• devcontainer.json passes JSON validation
• Using only ONE of: image, build.dockerfile, dockerComposeFile
• Features are compatible and ordered correctly
• Extensions use correct publisher.extension-name format

DV3: Environment Parity Issues

Symptoms: Works in VS Code, fails in Codespaces (or vice versa)

Common Issues:

Issue	Local VS Code	Codespaces
Docker socket	Usually available	Docker-in-Docker needed
Secrets	.env files work	Use Codespaces secrets
File watching	Native	May need polling

DV4: Multi-Service Complexity

Symptoms: Need database/cache/queue, services can't communicate

Interventions:

• Use Docker Compose integration
• Named volumes for persistence
• Health checks for service readiness
• Use assets/devcontainer-compose.md template

DV5: Dockerfile Issues

Symptoms: Build failures, huge images, no caching

Best Practices:

FROM mcr.microsoft.com/devcontainers/base:ubuntu

# Dependencies first (cached)
RUN apt-get update && apt-get install -y \
    build-essential && rm -rf /var/lib/apt/lists/*

# Copy deps then install (cached if deps unchanged)
COPY package*.json ./
RUN npm install

# Code last (changes frequently)
COPY . .

DV6: Devcontainer Validated

Indicators:

• Startup under 2 minutes
• Works in VS Code and Codespaces
• New developers productive in 30 minutes

Available Scripts

Script	Purpose	Usage
analyze-devcontainer.ts	Find issues and optimizations	deno run --allow-read scripts/analyze-devcontainer.ts
validate-dockerfile.ts	Check Dockerfile best practices	deno run --allow-read scripts/validate-dockerfile.ts
scan-image.ts	Vulnerability scanning (wraps Trivy)	deno run --allow-run scripts/scan-image.ts [image]

Anti-Patterns

The Kitchen Sink

Installing every tool "just in case" - 10+ minute startups. Fix: Start minimal. Add only when needed.

The postCreateCommand Overload

Everything in postCreateCommand - runs every time. Fix: Move stable operations to Dockerfile.

The Snowflake Container

Manual changes inside running containers. Fix: ALL changes go in config files.

Templates

• assets/devcontainer-simple.md - Basic single-container setup
• assets/devcontainer-dockerfile.md - Custom Dockerfile approach
• assets/devcontainer-compose.md - Multi-service setup

Related Skills

• system-design - Multi-service architecture informs Compose config
• pwa-development - Consistent environment for PWA toolchain