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/nodemodules ./nodemodules

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

Optimization Summary

TechniqueImpact

Alpine base image-70% size
Order COPY for cache hits-90% build time
Multi-stage buildRemoves build tools
.dockerignoreExcludes nodemodules, .git, docs
npm ci instead of npm installFaster, deterministic

nodemodules
.git
*.md
.env
.next