🐳 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!

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🧱 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.

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🪜 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.

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🧹 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.

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🏗️ 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%.

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🧼 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.

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📦 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.

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🐇 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.

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🔐 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.

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🪄 9. Clean Up Temporary Files 🧼

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

📝 Why?

Removes package cache after install to reduce bloat.

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🪛 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.

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🔒 11. Scan for Vulnerabilities 🧬

docker scan my-node-app

Or use:

• Docker Scout
• Trivy (Aqua Security)
• Snyk

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🧪 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

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🧠 Final Pro Tips 💡

Tip	Benefit
Use npm ci instead of npm install	Faster and more reliable
Group COPY steps wisely	Better cache usage
Avoid adding .env or secrets	Security risk
Label images (LABEL maintainer=...)	Better documentation
Run production builds with NODE_ENV=production	Removes dev dependencies

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📌 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"]

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