Docker Best Practices

Docker Best Practices

This skill provides current Docker best practices across all aspects of container development, deployment, and operation.

Image Best Practices

Base Image Selection

2025 Recommended Hierarchy:

1. Wolfi/Chainguard (cgr.dev/chainguard/*) - Zero-CVE goal, SBOM included
2. Alpine (alpine:3.19) - ~7MB, minimal attack surface
3. Distroless (gcr.io/distroless/*) - ~2MB, no shell
4. Slim variants (node:20-slim) - ~70MB, balanced

Key rules:

• Always specify exact version tags: node:20.11.0-alpine3.19
• Never use latest (unpredictable, breaks reproducibility)
• Use official images from trusted registries
• Match base image to actual needs

Dockerfile Structure

Optimal layer ordering (least to most frequently changing):

1. Base image and system dependencies
2. Application dependencies (package.json, requirements.txt, etc.)
3. Application code
4. Configuration and metadata

Rationale: Docker caches layers. If code changes but dependencies don't, cached dependency layers are reused, speeding up builds.

Example:

FROM python:3.12-slim

# 1. System packages (rarely change)
RUN apt-get update && apt-get install -y --no-install-recommends \
    gcc \
    && rm -rf /var/lib/apt/lists/*

# 2. Dependencies (change occasionally)
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# 3. Application code (changes frequently)
COPY . /app
WORKDIR /app

CMD ["python", "app.py"]

Multi-Stage Builds

Use multi-stage builds to separate build dependencies from runtime:

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

# Production stage
FROM node:20-alpine AS runtime
WORKDIR /app
# Only copy what's needed for runtime
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
USER node
CMD ["node", "dist/server.js"]

Benefits:

• Smaller final images (no build tools)
• Better security (fewer attack vectors)
• Faster deployment (smaller upload/download)

Layer Optimization

Combine commands to reduce layers and image size:

# Bad - 3 layers, cleanup doesn't reduce size
RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

# Good - 1 layer, cleanup effective
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl && \
    rm -rf /var/lib/apt/lists/*

.dockerignore

Always create .dockerignore to exclude unnecessary files:

# Version control
.git
.gitignore

# Dependencies
node_modules
__pycache__
*.pyc

# IDE
.vscode
.idea

# OS
.DS_Store
Thumbs.db

# Logs
*.log
logs/

# Testing
coverage/
.nyc_output
*.test.js

# Documentation
README.md
docs/

# Environment
.env
.env.local
*.local

Container Runtime Best Practices

Security

docker run \
  # Run as non-root
  --user 1000:1000 \
  # Drop all capabilities, add only needed ones
  --cap-drop=ALL \
  --cap-add=NET_BIND_SERVICE \
  # Read-only filesystem
  --read-only \
  # Temporary writable filesystems
  --tmpfs /tmp:noexec,nosuid \
  # No new privileges
  --security-opt="no-new-privileges:true" \
  # Resource limits
  --memory="512m" \
  --cpus="1.0" \
  my-image

Resource Management

Always set resource limits in production:

# docker-compose.yml