How do Docker layers work, and how would you optimize a Dockerfile for a Node.js application?

Docker Layers

Each RUN, COPY, ADD instruction creates a layer. Docker caches layers — if nothing changed, it reuses the cache.

# ❌ BAD: Code changes invalidate npm install cache
FROM node:20
WORKDIR /app
COPY . .                    # Everything copied → layer changes on any file change
RUN npm install             # Re-runs on every code change!
CMD ["node", "server.js"]
# Image size: 1.2GB, Build time: 8min

# ✅ GOOD: Cache npm install separately
FROM node:20-alpine          # Alpine = smaller base (180MB vs 1.1GB)
WORKDIR /app

# Copy dependency files FIRST (changes rarely)
COPY package.json package-lock.json ./
RUN npm ci --only=production  # Cached unless package.json changes

# Copy source code LAST (changes frequently)
COPY . .

CMD ["node", "server.js"]
# Image size: 250MB, Build time: 30s (with cache)

Multi-stage Build (production)

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

# Stage 2: Production (smaller, no dev dependencies)
FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules

EXPOSE 3000
CMD ["node", "dist/server.js"]
# Final image: ~180MB (no build tools, no dev deps)

Optimization Summary

Technique	Impact
Alpine base image	-70% size
Order COPY for cache hits	-90% build time
Multi-stage build	Removes build tools
.dockerignore	Excludes node_modules, .git, docs
npm ci instead of npm install	Faster, deterministic

node_modules
.git
*.md
.env
.next