Docker Multi-Stage Builds- Demo2

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We’ll build a simple Node.js app and serve it using Nginx.

Folder Structure

Create this structure:

multi-stage-demo/
├── Dockerfile
├── package.json
├── package-lock.json
├── server.js

Sample Node.js App

📄 package.json


{
  "name": "multi-stage-demo",
  "version": "1.0.0",
  "main": "server.js",
  "scripts": {
    "build": "echo 'Build step completed'",
    "start": "node server.js"
  },
  "dependencies": {
    "express": "^4.18.2"
  }
}

📄 server.js

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.send('Hello from Multi‑Stage Docker Build!');
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Dockerfile — Multi‑Stage Build (⭐ Core Part)

📄 Dockerfile

# -------------------------
# Stage 1: Build stage
# -------------------------

#Start a new image layer, Use the official Node.js version 18 image, Names this stage builder    
FROM node:18 AS builder

#Sets the working directory inside the container to /app
WORKDIR /app

#Copies: package.json, From your local machine Into /app inside the container
COPY package*.json ./

#Downloads all dependencies
RUN npm install

#Copies everything from current folder Into /app inside the container
COPY . .

#Builds the application
RUN npm run build


# -------------------------
# Stage 2: Runtime stage
# -------------------------
#Starts a brand new image, Based on Alpine Linux

FROM node:18-alpine

#Sets /app as working directory
WORKDIR /app

# Copy only required files from builder stage
COPY --from=builder /app .

#“This container listens on port 3000”
EXPOSE 3000

#Defines the default command when container starts
CMD ["npm", "start"]

What’s Happening Step by Step

✅ Stage 1: builder

  • Uses full Node image
  • Installs dependencies
  • Runs build command
  • Produces app files

✅ Stage 2: final image

  • Uses lighter Node Alpine image
  • Copies only needed output from Stage 1
  • No build tools included

🔍 Key Line Explained

Dockerfile

COPY –from=builder /app .
Show more lines

This copies files from the builder stage into the final image

🔄 Same Demo: WITHOUT vs WITH Multi‑Stage

WithoutWith Multi‑Stage
Single FROMMultiple FROM
Build tools remainBuild tools removed
Larger imageSmaller image
Slower deployFaster deploy

🔧 Build & Run the Image

Build Docker Image

docker build -t multi-stage-demo .

Run Container

docker run -p 3000:3000 multi-stage-demo

Open Browser

http://localhost:3000

✅ You’ll see:

Hello from Multi‑Stage Docker Build!
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