dockerfile-best-practices

Dockerfile Best Practices

Write secure, efficient, and maintainable Dockerfiles that produce minimal production images with proper caching, non-root users, and health checks.

When to Use

• User creates or modifies a Dockerfile
• User asks about reducing Docker image size
• User mentions container security or hardening
• User has slow Docker builds or cache invalidation issues
• User asks about COPY vs ADD or layer ordering

Core Patterns

Multi-Stage Builds

Separate build dependencies from runtime to minimize final image size.

# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --ignore-scripts
COPY src/ src/
COPY tsconfig.json ./
RUN npm run build

# Stage 2: Production
FROM node:20-alpine AS production
WORKDIR /app
RUN addgroup -g 1001 appgroup && \
    adduser -u 1001 -G appgroup -s /bin/sh -D appuser
COPY --from=builder --chown=appuser:appgroup /app/dist ./dist
COPY --from=builder --chown=appuser:appgroup /app/node_modules ./node_modules
COPY --from=builder --chown=appuser:appgroup /app/package.json ./
USER appuser
EXPOSE 3000
HEALTHCHECK --interval=30s --timeout=3s --retries=3 \
  CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "dist/index.js"]

Layer Caching Optimization

Order instructions from least to most frequently changed. Copy dependency manifests before source code so dependency installs are cached across builds.

FROM python:3.12-slim

WORKDIR /app

# System deps change rarely -- cache this layer
RUN apt-get update && \
    apt-get install -y --no-install-recommends libpq-dev && \
    rm -rf /var/lib/apt/lists/*

# Dependencies change occasionally -- cache this layer
COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt

# Source code changes frequently -- last layer
COPY . .

CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0"]

Non-Root User

Never run containers as root in production. Create a dedicated user with minimal permissions.

FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o /server ./cmd/server

FROM scratch
COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
COPY --from=builder /server /server
USER 65534:65534
ENTRYPOINT ["/server"]

.dockerignore

Always include a .dockerignore to prevent sending unnecessary files to the build context.

.git
.github
node_modules
dist
*.md
.env*
.vscode
.idea
docker-compose*.yml
Dockerfile*
coverage
__pycache__
*.pyc
.pytest_cache

Health Checks

Define health checks in the Dockerfile so orchestrators can monitor container health.

# HTTP health check
HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
  CMD curl -f http://localhost:8080/healthz || exit 1

# TCP health check (when curl is unavailable)
HEALTHCHECK --interval=30s --timeout=3s --retries=3 \
  CMD nc -z localhost 8080 || exit 1

Anti-Patterns

• Using latest tag: Always pin base image versions (node:20.11-alpine, not node:latest). Unpinned tags cause non-reproducible builds.
• Running as root: Never omit the USER instruction. Root in a container is root on the host if the container escapes.
• Using ADD instead of COPY: ADD auto-extracts archives and fetches URLs, which is unexpected. Use COPY for local files; use curl or wget explicitly for remote files.
• Installing dev dependencies in production: Use npm ci --omit=dev or pip install --no-dev in the final stage.
• Single-stage builds: Shipping compilers, build tools, and source code in production images wastes space and expands the attack surface.
• Not cleaning up apt/apk cache: Always add rm -rf /var/lib/apt/lists/* after apt-get install or use --no-cache with apk add.

Quick Reference

Practice	Do	Don't
Base image	node:20-alpine	node:latest
Copy files	COPY . .	ADD . .
User	USER 1001	(run as root)
Install deps	RUN npm ci	RUN npm install
Layer order	deps before source	source before deps
Secrets	--mount=type=secret	COPY .env .
Health	HEALTHCHECK CMD ...	(no health check)
Cache	rm -rf /var/lib/apt/lists/*	(leave cache)