# Docker Fundamentals for Developers

Why Docker?

Docker packages your application with everything it needs to run: the operating system, runtime, dependencies, and your code. This eliminates environment differences — what runs in Docker on your laptop runs identically in Docker on a server.

Your First Dockerfile

# Dockerfile for a Next.js application
# Multi-stage build: build stage + production stage

# Stage 1: Install dependencies
FROM node:20-alpine AS deps
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --only=production

# Stage 2: Build the application
FROM node:20-alpine AS builder
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 3: Production image
FROM node:20-alpine AS runner
WORKDIR /app
ENV NODE_ENV=production