🐳 Docker Image Optimization Guide — The Ultimate Cheat Sheet 🚀

Optimize your Docker images for faster builds, smaller size, better caching, and production readiness. Let’s go!

🧱 1. Use a Small & Specific Base Image 🐥

✅ Do This:

# Use lightweight Alpine variant FROM node:20-alpine 

❌ Avoid This:

# Heavy image — more layers, longer build times FROM ubuntu 

📝 Why?

• Smaller base = smaller image.
• Alpine images are ~5MB vs Ubuntu’s ~100MB.
• Smaller size = faster download, upload, deploy.

🪜 2. Order Instructions for Layer Caching 🔁

✅ Do This:

# Caches `npm install` unless package.json changes COPY package*.json ./ RUN npm install # Copy rest of the source after deps are installed COPY . . 

❌ Avoid This:

COPY . . # 👎 invalidates cache if any file changes RUN npm install 

📝 Why?

Docker caches layers. Changing a later step invalidates all subsequent layers. Put stable steps early for faster rebuilds.

🧹 3. Remove Unnecessary Files with .dockerignore 🚫

✅ Example .dockerignore:

node_modules Dockerfile .dockerignore .git npm-debug.log 

📝 Why?

It prevents Docker from copying unnecessary files into the build context. Less context = faster build and smaller image.

🏗️ 4. Use Multi-Stage Builds 🧪

✅ Example:

# Stage 1: Builder FROM node:20-alpine AS builder WORKDIR /app COPY package*.json ./ RUN npm install COPY . . # Stage 2: Runtime FROM node:20-alpine WORKDIR /app COPY --from=builder /app . EXPOSE 8000 CMD ["npm", "start"] 

📝 Why?

• Separate build dependencies from runtime.
• Final image contains only what’s needed to run the app.
• Reduces image size by up to 70%.

🧼 5. Remove Unused Dependencies 🧹

✅ Use --production for Node.js:

RUN npm ci --only=production 

OR

npm prune --production 

📝 Why?

Avoid bundling dev tools and test libraries into your production image.

📦 6. Combine RUN Commands 🔗

✅ Do This:

RUN apk add --no-cache bash curl && \  rm -rf /var/cache/apk/* 

❌ Don’t Do:

RUN apk add bash RUN apk add curl 

📝 Why?

Each RUN creates a layer. Combining reduces total layers = smaller image size.

🐇 7. Use --no-cache for Package Managers 📥

✅ Alpine:

RUN apk add --no-cache curl 

✅ APT (Debian/Ubuntu):

RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/* 

📝 Why?

Avoids unnecessary cache files and reduces image size.

🔐 8. Avoid Root User (Security Best Practice) 🔒

# Create a non-root user RUN addgroup -S appgroup && adduser -S appuser -G appgroup USER appuser 

📝 Why?

Running as root inside containers is risky. Use non-root users for security.

🪄 9. Clean Up Temporary Files 🧼

RUN npm install && \  npm cache clean --force 

📝 Why?

Removes package cache after install to reduce bloat.

🪛 10. Use Specific Tags (Not latest) 🎯

FROM node:20.11.1-alpine 

📝 Why?

Using latest can break builds if the base image updates and introduces changes. Always pin versions for reliability.

🔒 11. Scan for Vulnerabilities 🧬

docker scan my-node-app 

Or use:

• Docker Scout
• Trivy (Aqua Security)
• Snyk

🧪 12. Analyze Image Size and Layers 🔍

docker image inspect my-node-app docker history my-node-app 

Or use tools like:

• Dive: dive my-node-app
• DockerSlim: docker-slim build my-node-app

🧠 Final Pro Tips 💡

📌 Sample Optimized Dockerfile for Node.js App

# 👷 Stage 1: Build FROM node:20-alpine AS builder WORKDIR /app COPY package*.json ./ RUN npm ci COPY . . # ✨ Stage 2: Runtime FROM node:20-alpine WORKDIR /app COPY --from=builder /app . ENV NODE_ENV=production RUN npm prune --production EXPOSE 8000 CMD ["npm", "start"]