---

name: dockerfile-optimizer description: Analyzes and optimizes Dockerfiles to reduce image size, improve build time, and enhance security. Use when optimizing Docker images, reducing build times, or improving Dockerfile structure.

Dockerfile Optimizer

Optimize Docker images for size, speed, and security.

Quick Start

Analyze current Dockerfile:

Copydocker build -t myapp .
docker images myapp
# Note the size, then optimize

Instructions

Step 1: Analyze Current Dockerfile

Review for optimization opportunities:

• Base image choice
• Layer structure
• Build dependencies
• Caching strategy
• Security practices

Step 2: Optimize Base Image

Choose minimal base:

Copy# Before: Large base
FROM ubuntu:22.04  # ~77MB

# After: Minimal base
FROM alpine:3.18   # ~7MB
# or
FROM node:18-alpine  # ~170MB vs node:18 ~990MB

Use specific tags:

Copy# Bad: Unpredictable
FROM node:latest

# Good: Specific version
FROM node:18.17-alpine3.18

Step 3: Implement Multi-Stage Builds

For compiled languages:

Copy# Before: Single stage
FROM golang:1.21
WORKDIR /app
COPY . .
RUN go build -o app
CMD ["./app"]
# Result: ~800MB

# After: Multi-stage
FROM golang:1.21-alpine AS builder
WORKDIR /app
COPY . .
RUN go build -o app

FROM alpine:3.18
COPY --from=builder /app/app /app
CMD ["/app"]
# Result: ~15MB

Step 4: Optimize Layer Caching

Order by change frequency:

Copy# Bad: Code changes invalidate all layers
FROM node:18-alpine
COPY . .
RUN npm install
CMD ["node", "server.js"]

# Good: Dependencies cached separately
FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
CMD ["node", "server.js"]

Step 5: Combine and Minimize Layers

Reduce layer count:

Copy# Before: Multiple layers
RUN apt-get update
RUN apt-get install -y curl
RUN apt-get install -y git
RUN apt-get clean

# After: Single layer
RUN apt-get update && \
    apt-get install -y curl git && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

Step 6: Use .dockerignore

Create .dockerignore:

Copynode_modules
npm-debug.log
.git
.gitignore
README.md
.env
.DS_Store
*.md
.vscode
.idea
dist
build
coverage

Step 7: Remove Build Dependencies

Clean up after install:

Copy# Python example
RUN pip install --no-cache-dir -r requirements.txt

# Node example
RUN npm ci --only=production && npm cache clean --force

# Alpine example
RUN apk add --no-cache curl && \
    apk del build-dependencies

Step 8: Add Security Improvements

Run as non-root:

Copy# Create user
RUN addgroup -g 1001 -S appuser && \
    adduser -u 1001 -S appuser -G appuser

# Switch to user
USER appuser

# Or use existing user
USER nobody

Step 9: Verify Optimization

Check image size:

Copydocker images myapp
docker history myapp

Analyze layers:

Copydocker history myapp --no-trunc

Use dive tool:

Copydive myapp

Optimization Patterns

Node.js Optimization:

CopyFROM node:18-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production

FROM node:18-alpine
WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY . .
USER node
EXPOSE 3000
CMD ["node", "server.js"]

Python Optimization:

CopyFROM python:3.11-slim AS builder
WORKDIR /app
COPY requirements.txt .
RUN pip install --user --no-cache-dir -r requirements.txt

FROM python:3.11-slim
WORKDIR /app
COPY --from=builder /root/.local /root/.local
COPY . .
ENV PATH=/root/.local/bin:$PATH
USER nobody
CMD ["python", "app.py"]

Go Optimization:

CopyFROM golang:1.21-alpine AS builder
WORKDIR /app
COPY go.* ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM scratch
COPY --from=builder /app/app /app
COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
EXPOSE 8080
CMD ["/app"]

Common Issues

Issue: Large image size

• Use alpine or slim base images
• Implement multi-stage builds
• Remove build dependencies
• Use .dockerignore

Issue: Slow builds

• Optimize layer caching
• Order commands by change frequency
• Use build cache mounts
• Parallelize where possible

Issue: Security vulnerabilities

• Use minimal base images
• Keep images updated
• Run as non-root
• Scan with security tools

Issue: Build cache not working

• Check command order
• Avoid COPY . . early
• Use specific COPY commands
• Check .dockerignore

Advanced

For detailed information, see:

• Multi-Stage Builds - Advanced multi-stage patterns
• Layer Caching - Caching strategies and optimization
• Base Images - Base image selection guide