Dockerfile Best Practices

Comprehensive guide for creating optimized, secure, and fast Docker images using modern BuildKit features.

Quick Start Workflow

When working with Dockerfiles, follow this decision tree:

1. Choose your language/framework → Use appropriate template below
2. Apply security hardening → Non-root user, pin versions, secrets management
3. Optimize for cache → Separate deps from code, use cache mounts
4. Multi-stage if needed → Separate build from runtime for compiled languages
5. Review with analyzer → Run scripts/analyze_dockerfile.py

Essential Rules (Always Apply)

1. Start with BuildKit syntax:

   dockerfileCopy

   # syntax=docker/dockerfile:1
   

2. Pin runtime versions, not OS versions:

   dockerfileCopy

   # ✅ GOOD - Pin runtime, let OS update
   FROM python:3.12-slim
   
   # ❌ BAD - Pins OS version (bookworm), prevents security updates
   FROM python:3.12-slim-bookworm
   

   Why? OS versions update with security patches. Pin runtime (python:3.12) for reproducible behavior.

3. Create .dockerignore: Use template from assets/dockerignore-template

4. Use cache mounts for package managers:

   dockerfileCopy

   RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
   

5. APT cache setup (before any apt operations):

   dockerfileCopy

   RUN rm -f /etc/apt/apt.conf.d/docker-clean; \
       echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
   

6. Never use ARG/ENV for secrets:

   dockerfileCopy

   RUN --mount=type=secret,id=api_key curl -H "Authorization: $(cat /run/secrets/api_key)" https://api.example.com
   

7. Use non-root user with safe UID/GID:

   dockerfileCopy

   # Let system auto-assign (simple)
   RUN groupadd -r app && useradd -r -g app app
   
   # Or use UID/GID >10000 for consistency across environments
   RUN groupadd -r -g 10001 app && useradd -r -u 10001 -g app app
   

Language-Specific Templates

Python (with uv - Recommended)

For detailed Python/uv integration, see references/uv_integration.md.

# syntax=docker/dockerfile:1

FROM python:3.12-slim
COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/

WORKDIR /app

# Install dependencies (separate layer for caching)
RUN --mount=type=cache,target=/root/.cache/uv \
    --mount=type=bind,source=uv.lock,target=uv.lock \
    --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
    uv sync --locked --no-install-project

# Copy and install project
COPY . .
RUN --mount=type=cache,target=/root/.cache/uv \
    uv sync --locked

# Security: non-root user with UID/GID >10000
RUN groupadd -r -g 10001 app && \
    useradd -r -u 10001 -g app app && \
    chown -R app:app /app
USER app

ENV PATH="/app/.venv/bin:$PATH"
CMD ["python", "-m", "myapp"]

Node.js

# syntax=docker/dockerfile:1

FROM node:20-alpine

WORKDIR /app

# Install dependencies
COPY package.json yarn.lock ./
RUN --mount=type=cache,target=/root/.yarn \
    yarn install --frozen-lockfile --production

# Copy source
COPY . .

# Security: non-root user with UID/GID >10000
RUN addgroup -g 10001 app && \
    adduser -u 10001 -G app -S app && \
    chown -R app:app /app
USER app

EXPOSE 3000
CMD ["node", "index.js"]

Go (Multi-stage)

# syntax=docker/dockerfile:1

# Build stage
FROM golang:1-alpine AS builder
WORKDIR /app

COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod \
    go mod download

COPY . .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o main

# Runtime stage
FROM alpine:3
RUN addgroup -g 10001 app && adduser -u 10001 -G app -S app
USER app
COPY --from=builder /app/main /main
ENTRYPOINT ["/main"]

Debian-based (with APT cache)

# syntax=docker/dockerfile:1

FROM debian:stable-slim

# Configure APT for cache reuse
RUN rm -f /etc/apt/apt.conf.d/docker-clean; \
    echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache

WORKDIR /app

# Install dependencies with cache mount
RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
    --mount=type=cache,target=/var/lib/apt,sharing=locked \
    apt-get update && \
    apt-get install -y --no-install-recommends curl ca-certificates

# Copy application
COPY . .

# Security: non-root user with UID/GID >10000
RUN groupadd -r -g 10001 app && \
    useradd -r -u 10001 -g app app && \
    chown -R app:app /app
USER app

CMD ["./app"]

PHP (with Composer)

# syntax=docker/dockerfile:1

FROM php:8-fpm-alpine

WORKDIR /app

# Install Composer
COPY --from=composer:latest /usr/bin/composer /usr/bin/composer

# Install dependencies
COPY composer.json composer.lock ./
RUN --mount=type=cache,target=/tmp/cache \
    composer install --no-dev --optimize-autoloader --no-scripts

# Copy source
COPY . .
RUN composer dump-autoload --optimize

# Security: non-root user with UID/GID >10000
RUN addgroup -g 10001 app && \
    adduser -u 10001 -G app -S app && \
    chown -R app:app /app
USER app

EXPOSE 9000
CMD ["php-fpm"]

Security Checklist

• Use specific version tags for base images
• Use minimal base image (alpine, slim, distroless)
• Create and use non-root user
• Never expose secrets via ARG/ENV
• Use RUN --mount=type=secret for sensitive data
• Add HEALTHCHECK instruction
• Scan image for vulnerabilities (docker scan)

Performance Checklist

• Add BuildKit syntax directive
• Create comprehensive .dockerignore
• Order instructions: manifests → deps → code
• Use --mount=type=cache for all package managers
• Implement multi-stage builds for compiled languages
• Chain RUN commands with &&
• Clean up in same RUN instruction

Size Optimization

Quick wins:

1. Use alpine or slim variants
2. Multi-stage builds
3. Remove unnecessary files in same layer
4. Use --no-install-recommends with apt
5. Consider distroless for runtime

Common Patterns

Intermediate Layers (Faster Rebuilds)

Separate dependency installation from code copy:

# Install deps first (rarely changes)
COPY package.json package-lock.json ./
RUN npm ci --production

# Copy code (changes frequently)
COPY . .

Remote Cache (CI/CD)

docker buildx build \
  --cache-from=type=registry,ref=myregistry.com/app:cache \
  --cache-to=type=registry,ref=myregistry.com/app:cache,mode=max \
  --push \
  -t myregistry.com/app:latest .

Secret Management

# Build with secret
docker buildx build --secret id=api_key,src=./key.txt .

# Or from environment
docker buildx build --secret id=api_key,env=API_KEY .

Tools and Scripts

Analyze Dockerfile

python scripts/analyze_dockerfile.py ./Dockerfile

Detects common anti-patterns:

• Missing BuildKit syntax
• Using :latest tags
• ADD instead of COPY
• Missing cache mounts
• Secrets in ARG/ENV
• Missing non-root USER
• apt-get without cleanup

Generate .dockerignore

cp assets/dockerignore-template .dockerignore

Docker Compose Best Practices

For multi-container applications, follow modern Compose practices:

Key Rules

1. Don't use version: field - Deprecated since Compose V2

   yamlCopy

   # ✅ GOOD - No version field
   services:
     app:
       image: myapp:1.0.0
   

2. Never use container_name: - Prevents scaling and parallel environments

   yamlCopy

   # ✅ GOOD - Let Compose generate names
   services:
     app:
       image: myapp:1.0.0
       # No container_name - allows scaling with --scale
   
   # Use project names for environment isolation:
   # docker compose -p myapp-dev up
   # docker compose -p myapp-test up
   

3. Use specific image tags - Not :latest

4. Define health checks - For service dependencies

5. Set resource limits - Prevent resource exhaustion

For complete Compose guide, see references/compose_best_practices.md.

Reference Documentation

For detailed information, consult these references:

1. references/optimization_guide.md - Complete optimization guide with BuildKit, caching, multi-stage builds
2. references/best_practices.md - Checklist of all best practices with impact levels
3. references/examples.md - Real-world before/after optimization examples
4. references/uv_integration.md - Python with uv package manager (recommended for Python)
5. references/compose_best_practices.md - Docker Compose modern practices (no version:, no container_name:)

Common Issues and Solutions

Slow builds

• Add cache mounts: RUN --mount=type=cache,target=...
• Optimize layer ordering: deps before code
• Use remote cache in CI/CD

Large images

• Use multi-stage builds
• Switch to alpine/slim base images
• Clean up in same RUN instruction
• Remove dev dependencies

Security concerns

• Pin versions with SHA256
• Use non-root USER
• Never use ARG/ENV for secrets
• Use BuildKit secret mounts
• Scan with docker scan

Cache invalidation

• Separate dependency installation from code
• Use --mount=type=bind for manifests
• Order instructions correctly

Examples by Use Case

CLI Tool

# syntax=docker/dockerfile:1
FROM python:3.12-alpine
COPY --from=ghcr.io/astral-sh/uv:latest /uv /bin/
WORKDIR /app
RUN --mount=type=cache,target=/root/.cache/uv \
    --mount=type=bind,source=uv.lock,target=uv.lock \
    --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
    uv sync --locked --no-install-project
COPY . .
RUN --mount=type=cache,target=/root/.cache/uv uv sync --locked
ENV PATH="/app/.venv/bin:$PATH"
ENTRYPOINT ["mytool"]

Web API

# syntax=docker/dockerfile:1
FROM node:20-alpine
WORKDIR /app
COPY package.json yarn.lock ./
RUN --mount=type=cache,target=/root/.yarn yarn install --frozen-lockfile --production
COPY . .
RUN addgroup -g 10001 app && \
    adduser -u 10001 -G app -S app && \
    chown -R app:app /app
USER app
EXPOSE 3000
HEALTHCHECK --interval=30s CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "server.js"]

Static Site

# syntax=docker/dockerfile:1
FROM node:20-alpine AS builder
WORKDIR /app
COPY package.json yarn.lock ./
RUN --mount=type=cache,target=/root/.yarn yarn install --frozen-lockfile
COPY . .
RUN yarn build

FROM nginx:alpine
COPY --from=builder /app/dist /usr/share/nginx/html
EXPOSE 80

Build Commands Reference

# Basic build
docker buildx build -t myapp:latest .

# With cache from registry
docker buildx build \
  --cache-from=type=registry,ref=registry.com/myapp:cache \
  --cache-to=type=registry,ref=registry.com/myapp:cache,mode=max \
  -t myapp:latest .

# With secrets
docker buildx build \
  --secret id=api_key,src=./key.txt \
  -t myapp:latest .

# Multi-platform
docker buildx build \
  --platform linux/amd64,linux/arm64 \
  -t myapp:latest --push .

# Target specific stage
docker buildx build --target builder -t myapp:builder .

Next Steps

After creating your Dockerfile:

1. Run analyzer: python scripts/analyze_dockerfile.py Dockerfile
2. Test locally: docker buildx build -t test .
3. Check size: docker images test
4. Scan for vulnerabilities: docker scout cves test
5. Profile build: docker buildx build --progress=plain . 2>&1 | less