Devcontainer

devcontainer

When to use

Use this skill when working with DevContainer configuration, GitHub Codespaces setup, or development environment standardization.

Do NOT use when:

• Local Docker setup without Codespaces (use docker skill)
• Production deployment (use aws-infrastructure skill)

Procedure: Modify DevContainer

1. Gather context — read .devcontainer/devcontainer.json, check .devcontainer/ for env files and docs, check agents/overrides/skills/devcontainer.md for project-specific overrides.
2. Identify change type — classify: image change, feature addition, secret addition, extension change, or env var change.
3. Make the change — edit devcontainer.json (or related files). Follow conventions below for secrets, features, and environment variables.
4. Build and verify — run devcontainer build to confirm the container builds. Check that extensions load and ports forward correctly.
5. Document — if adding a new secret or dependency, update the onboarding docs in .devcontainer/.

Architecture

Custom image

DevContainers typically use a pre-built custom image hosted on a container registry (GHCR, ECR, Docker Hub). Read devcontainer.json for the image URL.

The image should include:

• Language runtime (PHP, Node.js, Python, etc.)
• Package managers (Composer, npm, etc.)
• Common development tools

Features (installed on top of the image)

Common features:

Secrets management

Secrets can be managed via:

• File mounts: .devcontainer/.secrets/<NAME> → /run/secrets/<NAME>
• GitHub Codespaces secrets: configured in the repository settings
• Environment variables: in .devcontainer/devcontainer.env

Read devcontainer.json for the actual secret definitions and requirements.

Workspace

• Workspace folder: typically /workspace or /workspaces/{repo-name}
• Mount type: bind mount from local workspace
• Container name and hostname: defined in devcontainer.json

IDE integration

• VS Code Live Share for collaborative development
• IDE extensions can be pre-configured in devcontainer.json

Conventions

Adding new secrets

1. Add the secret definition to devcontainer.json → secrets section.
2. Add a bind mount from .devcontainer/.secrets/<NAME> to /run/secrets/<NAME>.
3. Document the secret with description and documentationUrl.
4. Mark as required or optional.

Modifying the base image

• If the base image is maintained in a separate repository, do NOT change the image tag without coordination.
• Prefer adding features over modifying the base image.

Environment variables

• Runtime env vars go in .devcontainer/devcontainer.env.
• Secrets go in .devcontainer/.secrets/ (gitignored).
• Do NOT hardcode secrets in devcontainer.json.

Output format

1. Updated devcontainer.json or related configuration files
2. Summary of changes and rebuild requirements

Auto-trigger keywords

• DevContainer
• Codespaces
• dev environment
• container setup

Validate

• Verify the DevContainer builds successfully (devcontainer build).
• Confirm all required extensions and features are installed.
• Check that port forwarding and volume mounts work as expected.

Gotcha

• DevContainer rebuilds are slow — test configuration changes incrementally, not all at once.
• Secrets in devcontainer.json are visible in version control — use Codespaces secrets instead.
• Extensions in devcontainer.json install on EVERY rebuild — keep the list short.

Do NOT

• Do NOT commit secret files — they should be gitignored.
• Do NOT change the workspace mount path without updating all related configs.
• Do NOT remove required secrets without checking which services depend on them.
• Do NOT switch base images without team approval.

Related

• Skill: docker — Docker setup, multi-stage Dockerfile, compose services
• Skill: traefik — local reverse proxy with real domains and HTTPS
• Rule: docker-commands.md — commands run inside Docker