docker-containerization

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• SKILL.md

SKILL.md (first lines)

# Docker Containerization

Build efficient, secure container images and compose multi-service architectures.

## Multi-Stage Builds

### Python Application

```dockerfile
# Stage 1: Build
FROM python:3.12-slim AS builder
WORKDIR /app
COPY pyproject.toml .
RUN pip install --no-cache-dir --prefix=/install .

# Stage 2: Runtime
FROM python:3.12-slim
COPY --from=builder /install /usr/local
COPY src/ /app/src/
WORKDIR /app
USER nobody
EXPOSE 8000
CMD ["python", "-m", "uvicorn", "src.app:app", "--host", "0.0.0.0", "--port", "8000"]
```

### Node.js Application

```dockerfile
# Stage 1: Dependencies
FROM node:20-alpine AS deps
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev

# Stage 2: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 3: Runtime
FROM node:20-alpine
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
USER node
EXPOSE 3000
CMD ["node", "dist/index.js"]
```

## Image Optimization

### Layer Ordering

Order instructions from least to most frequently changing:

```dockerfile
FROM python:3.12-slim
# 1. System deps (rarely change)
RUN apt-get update && apt-get install -y --no-install-recommends \
    libpq-dev && rm -rf /var/lib/apt/lists/*
# 2. Python deps (change occasionally)
COPY pyproject.toml .
RUN pip install --no-cache-dir .
# 3. Application code (changes often)
COPY src/ ./src/
```

### Size Reduction

| Technique | Savings |
|-----------|---------|
| Alpine/slim base | 50-80% |
| Multi-stage builds | 40-70% |
| `--no-cache-dir` for pip | 10-20% |
| `.dockerignore` | Variable |
| Combine RUN layers | 5-15% |

### .dockerignore

```
.git
.venv
__pycache__
*.pyc
node_modules
.env
*.md
tests/
docs/
.build/
```

## Docker Compose

### Multi-Service Architecture

```yaml
services:
  api:
    build: .
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/app
      - REDIS_URL=redis://cache:6379
    depends_on:
      db:
        condition: service_healthy
      cache:
        condition: service_started
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
      interval: 30s
      timeout: 5s
      retries: 3

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