Day 2 - Fix the Slow Docker Build

Description

You've inherited a Dockerfile from a previous developer, and it's painfully slow. Every code change triggers a full rebuild that takes several minutes. Your team is frustrated with the slow feedback loop during development.

Task

Rewrite the Dockerfile so the build time is reduced by at least 50%.

Requirements:

Maintain the same functionality
Reduce build time by minimum 50%
Optimize layer caching
Follow Docker best practices

Target

Build Time Reduction: 50% or more
Cache Hit Rate: Maximize layer reuse
First Build: May be similar, but rebuilds should be fast

Sample App

Slow Dockerfile (Before)

FROM node:20

# Install everything first (breaks cache on any file change)
WORKDIR /app
COPY . .

# Install dependencies (runs every time anything changes)
RUN npm install

# Build the application
RUN npm run build

# Install production dependencies
RUN npm prune --production

EXPOSE 3000

CMD ["node", "dist/index.js"]

package.json

{
  "name": "advent-app",
  "version": "1.0.0",
  "scripts": {
    "build": "tsc",
    "start": "node dist/index.js"
  },
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "@types/express": "^4.17.17",
    "@types/node": "^20.0.0",
    "typescript": "^5.0.0"
  }
}

View Solution

Solution

Optimized Dockerfile (After)

FROM node:20-alpine AS builder

WORKDIR /app

# Copy dependency files first (better caching)
COPY package*.json ./

# Install all dependencies (including dev deps for build)
RUN npm ci --only=production=false

# Copy source code
COPY tsconfig.json ./
COPY src ./src

# Build application
RUN npm run build

# Production stage
FROM node:20-alpine

WORKDIR /app

# Copy package files
COPY package*.json ./

# Install only production dependencies
RUN npm ci --only=production

# Copy built application from builder
COPY --from=builder /app/dist ./dist

# Create non-root user
RUN addgroup -g 1001 appgroup && \
    adduser -D -u 1001 -G appgroup appuser && \
    chown -R appuser:appgroup /app

USER appuser

EXPOSE 3000

CMD ["node", "dist/index.js"]

.dockerignore

node_modules
dist
.git
.gitignore
README.md
.env
.env.local
npm-debug.log
.DS_Store
*.md
.vscode
.idea
coverage
.nyc_output

Explanation

What Was Wrong?

The original Dockerfile had several performance issues:

Poor Layer Ordering: Copying all files before installing dependencies
No Cache Optimization: Every code change invalidated the dependency layer
Missing .dockerignore: Copying unnecessary files (node_modules, .git)
No Multi-Stage Build: Including build tools in final image
npm install vs npm ci: Not using deterministic installs

Optimizations Applied

1. Dependency Layer Caching

# ❌ Bad: Code changes invalidate dependency cache
COPY . .
RUN npm install

# ✅ Good: Dependencies cached separately
COPY package*.json ./
RUN npm ci
COPY . .

2. Multi-Stage Builds

# Build stage has dev dependencies
FROM node:20-alpine AS builder
RUN npm ci

# Production stage only has runtime dependencies
FROM node:20-alpine
RUN npm ci --only=production

3. Using npm ci Instead of npm install

npm ci is faster and more reliable
Uses package-lock.json for deterministic installs
Removes existing node_modules first

4. .dockerignore File

Excludes unnecessary files from build context:

Reduces context size
Faster uploads to Docker daemon
Prevents accidental inclusion of secrets

Build Time Comparison

Scenario	Before	After	Improvement
First build	120s	100s	17% faster
Code change	120s	15s	87% faster
Dependency change	120s	80s	33% faster

Try to solve the challenge yourself first!

Click "Reveal Solution" when you're ready to see the answer.

Result

You should achieve:

✅ 50%+ faster rebuilds on code changes
✅ Cached dependency layers
✅ Smaller final image size
✅ Faster CI/CD pipelines

Validation

Test Build Performance

# First build (cold cache)
time docker build -t advent-day2:v1 .

# Make a small code change
echo "console.log('test');" >> src/index.ts

# Rebuild (should be much faster)
time docker build -t advent-day2:v2 .

# Check cache usage
docker build -t advent-day2:v3 . --progress=plain
# Look for "CACHED" messages

Measure Cache Efficiency

# Build with cache stats
docker build -t advent-day2:latest . \
  --progress=plain 2>&1 | grep -c "CACHED"

Advanced Optimizations

BuildKit Features

# syntax=docker/dockerfile:1.4

# Use build cache mounts
RUN --mount=type=cache,target=/root/.npm \
    npm ci

Parallel Multi-Stage Builds

FROM base AS dependencies
RUN npm ci

FROM base AS test
COPY --from=dependencies /app/node_modules ./node_modules
RUN npm test

FROM base AS build
COPY --from=dependencies /app/node_modules ./node_modules
RUN npm run build