Dev Containers & Nix Environments

Reproducible, portable development environments that eliminate environment drift.

When to Use This Skill

Use this skill when:

• Onboarding new developers (zero-to-productive in minutes)
• Standardizing toolchains across a team
• Eliminating "works on my machine" problems
• Setting up CI environments that match local dev
• Creating ephemeral, disposable dev environments

Dev Containers

Basic Configuration

// .devcontainer/devcontainer.json
{
  "name": "My Project",
  "image": "mcr.microsoft.com/devcontainers/base:ubuntu-22.04",
  "features": {
    "ghcr.io/devcontainers/features/node:1": { "version": "20" },
    "ghcr.io/devcontainers/features/python:1": { "version": "3.12" },
    "ghcr.io/devcontainers/features/docker-in-docker:2": {},
    "ghcr.io/devcontainers/features/kubectl-helm-minikube:1": {}
  },
  "forwardPorts": [3000, 5432],
  "postCreateCommand": "npm install",
  "customizations": {
    "vscode": {
      "extensions": [
        "dbaeumer.vscode-eslint",
        "esbenp.prettier-vscode",
        "ms-python.python"
      ],
      "settings": {
        "editor.formatOnSave": true
      }
    }
  }
}

Docker Compose Dev Container

// .devcontainer/devcontainer.json
{
  "name": "Full Stack Dev",
  "dockerComposeFile": "docker-compose.yml",
  "service": "app",
  "workspaceFolder": "/workspace",
  "forwardPorts": [3000, 5432, 6379],
  "postCreateCommand": "npm install && npx prisma migrate dev"
}

# .devcontainer/docker-compose.yml
services:
  app:
    build:
      context: ..
      dockerfile: .devcontainer/Dockerfile
    volumes:
      - ..:/workspace:cached
    command: sleep infinity
    depends_on: [db, redis]

  db:
    image: postgres:16
    environment:
      POSTGRES_DB: dev
      POSTGRES_USER: dev
      POSTGRES_PASSWORD: dev
    volumes:
      - pgdata:/var/lib/postgresql/data
    ports:
      - "5432:5432"

  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"

volumes:
  pgdata:

Custom Dockerfile

# .devcontainer/Dockerfile
FROM mcr.microsoft.com/devcontainers/base:ubuntu-22.04

# System dependencies
RUN apt-get update && apt-get install -y \
    build-essential \
    curl \
    git \
    jq \
    unzip \
    && rm -rf /var/lib/apt/lists/*

# Install project-specific tools
RUN curl -fsSL https://get.opentofu.org/install-opentofu.sh -o /tmp/install-opentofu.sh \
    && sh /tmp/install-opentofu.sh -s -- --install-method standalone \
    && rm /tmp/install-opentofu.sh
RUN curl -LO "https://dl.k8s.io/release/$(curl -sL https://dl.k8s.io/release/stable.txt)/bin/linux/amd64/kubectl" \
    && install kubectl /usr/local/bin/

# Non-root user setup
USER vscode
WORKDIR /workspace

Nix Flakes

Basic Flake

# flake.nix
{
  description = "Project development environment";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        devShells.default = pkgs.mkShell {
          buildInputs = with pkgs; [
            # Languages
            nodejs_20
            python312
            go_1_22
            rustc
            cargo

            # Tools
            docker-compose
            kubectl
            kubernetes-helm
            opentofu
            awscli2
            jq
            yq-go

            # Databases
            postgresql_16
            redis
          ];

          shellHook = ''
            echo "Dev environment loaded"
            export PROJECT_ROOT=$(pwd)
            export PATH="$PROJECT_ROOT/node_modules/.bin:$PATH"
          '';
        };
      }
    );
}

# Enter the dev shell
nix develop

# Or run a single command
nix develop --command bash -c "node --version && go version"

# Build and run
nix build
nix run

Pin Dependencies

# Lock flake inputs for reproducibility
nix flake lock
nix flake update  # Update all inputs

# Update a specific input
nix flake lock --update-input nixpkgs

Devbox (Nix Made Simple)

Devbox wraps Nix with a friendlier interface:

# Install Devbox
curl -fsSL https://get.jetify.com/devbox -o /tmp/install-devbox.sh && sh /tmp/install-devbox.sh && rm /tmp/install-devbox.sh

# Initialize project
devbox init

# Add packages
devbox add nodejs@20 python@3.12 postgresql@16
devbox add go@1.22 kubectl helm

# Enter shell
devbox shell

# Run commands without entering shell
devbox run node --version

devbox.json Configuration

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/main/.schema/devbox.schema.json",
  "packages": [
    "nodejs@20",
    "python@3.12",
    "go@1.22",
    "kubectl@1.29",
    "kubernetes-helm@3.14",
    "opentofu@1.8",
    "awscli2@2.15",
    "jq@1.7",
    "postgresql@16",
    "redis@7"
  ],
  "env": {
    "PROJECT_ROOT": "$PWD",
    "DATABASE_URL": "postgresql://localhost:5432/dev"
  },
  "shell": {
    "init_hook": [
      "echo 'Dev environment ready'",
      "npm install --silent 2>/dev/null || true"
    ],
    "scripts": {
      "dev": "npm run dev",
      "test": "npm test",
      "db:start": "pg_ctl -D .devbox/virtenv/postgresql/data start",
      "db:stop": "pg_ctl -D .devbox/virtenv/postgresql/data stop",
      "db:migrate": "npx prisma migrate dev"
    }
  }
}

# Run project scripts
devbox run dev
devbox run test
devbox run db:start

# Generate direnv integration (auto-activate on cd)
devbox generate direnv

# Generate Dockerfile from devbox config
devbox generate dockerfile

Devbox + direnv (Auto-Activate)

# Install direnv
devbox add direnv

# Generate .envrc
devbox generate direnv

# Allow direnv
direnv allow

# .envrc (auto-generated)
eval "$(devbox generate direnv --print-envrc)"

Now cd-ing into the project automatically loads the environment.

CI/CD Integration

GitHub Actions with Devbox

# .github/workflows/ci.yml
name: CI
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: jetify-com/devbox-install-action@v0.11.0
        with:
          enable-cache: true
      - run: devbox run test
      - run: devbox run lint

GitHub Actions with Nix

name: CI
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: cachix/install-nix-action@v26
        with:
          nix_path: nixpkgs=channel:nixos-unstable
      - uses: cachix/cachix-action@v14
        with:
          name: my-project
          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
      - run: nix develop --command bash -c "npm ci && npm test"

GitHub Codespaces

// .devcontainer/devcontainer.json — works in Codespaces
{
  "name": "Codespaces Dev",
  "image": "mcr.microsoft.com/devcontainers/universal:2",
  "features": {
    "ghcr.io/devcontainers/features/node:1": { "version": "20" }
  },
  "postCreateCommand": "npm install",
  "portsAttributes": {
    "3000": { "label": "App", "onAutoForward": "openBrowser" },
    "5432": { "label": "Postgres", "onAutoForward": "ignore" }
  }
}

Comparison

Best Practices

• Pin all tool versions explicitly — never use latest
• Commit lock files (flake.lock, devbox.lock, etc.)
• Use direnv for automatic environment activation
• Cache Nix store in CI (Cachix or GitHub cache)
• Document setup in README: devbox shell or nix develop
• Keep dev environment close to production (same Node/Python versions)

Troubleshooting

Related Skills

• docker-management (docker-management) — Container image optimization
• github-actions (github-actions) — CI/CD pipeline setup
• linux-administration (linux-administration) — System-level tooling

Limitations

• Guidance executes against real environments: confirm target, blast radius, and rollback plan before applying anything.
• Never deploy to production without explicit approval. Docs-only import: upstream scripts and templates not bundled.

Example

git status && git diff --stat
kubectl diff -f manifest.yaml

Adapted from BagelHole/DevOps-Security-Agent-Skills (MIT); frontmatter, When to Use/Limitations, and safety boundaries added for upstream compliance. Docs-only import: helper scripts and templates not bundled.