dockerfile-slim

Analyze and optimize Dockerfiles - reduce image size by up to 80%

Problem

Your Docker image is 2GB. Deploys take forever. Registry storage costs money.

Solution

dockerfile-slim analyzes your Dockerfile and provides actionable optimization suggestions with estimated size and cost savings.

Features

• Instant analysis - No Docker build required
• Base image alternatives - Suggests smaller base images (e.g., node → node:alpine)
• Multi-stage build detection - Identifies missing multi-stage opportunities
• Layer optimization - Finds inefficient layer patterns
• Cache optimization - Detects poor layer caching strategies
• Cost estimation - Shows monthly registry cost savings
• Auto-fix - Generates optimized Dockerfiles
• .dockerignore generation - Prevents bloat from unnecessary files

Installation

# Run directly with npx (recommended)
npx dockerfile-slim

# Or install globally
npm install -g dockerfile-slim

# Or as dev dependency
npm install --save-dev dockerfile-slim

Usage

Basic Analysis

# Analyze Dockerfile in current directory
npx dockerfile-slim

# Analyze specific Dockerfile
npx dockerfile-slim ./path/to/Dockerfile

# Output as JSON (for CI/CD)
npx dockerfile-slim --json

Generate Optimized Dockerfile

# Generate and display optimized Dockerfile
npx dockerfile-slim --fix

# Save to file
npx dockerfile-slim --fix --output Dockerfile.optimized

# Also generate .dockerignore
npx dockerfile-slim --fix --dockerignore

Example Output

📦 Dockerfile Analysis
──────────────────────────────────────────────────

File:             ./Dockerfile
Base Image:       node:18
Estimated Size:   1.2 GB
Layer Count:      8
Efficiency Score: 35/100

💡 Found 5 optimization opportunities:

🔴 Critical

🔴 Smaller base image available
   Switch from node:18 to node:18-alpine
   → Potential savings: 965 MB
   → Cost savings: ~$5.30/month

🔴 Missing multi-stage build (Node.js)
   Node.js projects should use multi-stage builds
   → Potential savings: 800 MB
   → Cost savings: ~$4.40/month

🟠 High Priority

🟠 apt-get without cleanup
   apt-get install without cleaning cache adds ~50-200MB
   Line 5
   → Potential savings: 100 MB
   → Add cleanup in same RUN: && rm -rf /var/lib/apt/lists/*

🟠 Missing .dockerignore
   Without .dockerignore, unnecessary files may be included
   → Potential savings: 200 MB

🟡 Medium Priority

🟡 npm cache not cleaned
   npm cache can add 50-100MB
   → Potential savings: 50 MB
   → Add npm cache clean --force after install

──────────────────────────────────────────────────

Summary                      Value
───────────────────────────────────────
Total potential savings      2.0 GB
Estimated cost savings       $11.20/month
Potential size reduction     80%

💡 Run with --fix to generate an optimized Dockerfile

Generated Dockerfile Example

Running npx dockerfile-slim --fix generates:

# syntax=docker/dockerfile:1

# ---- Build Stage ----
FROM node:20-slim AS builder

WORKDIR /app

# Copy package files first for better layer caching
COPY package.json package-lock.json* ./

# Install ALL dependencies (including devDependencies for build)
RUN npm ci

# Copy source code
COPY . .

# Build the application
RUN npm run build

# ---- Production Stage ----
FROM node:20-slim AS production

# Create non-root user for security
RUN groupadd --gid 1001 nodejs && \
    useradd --uid 1001 --gid nodejs --shell /bin/bash --create-home nodejs

WORKDIR /app

# Copy package files
COPY package.json package-lock.json* ./

# Install production dependencies only
RUN npm ci --omit=dev && \
    npm cache clean --force

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

# Set ownership to non-root user
RUN chown -R nodejs:nodejs /app

USER nodejs

EXPOSE 3000

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

Comparison with Other Tools

| Tool | Analysis | Auto-Fix | Cost Est. | npm Native | Maintained | |------|----------|----------|-----------|------------|------------| | dockerfile-slim | ✅ | ✅ | ✅ | ✅ | ✅ | | Hadolint | ✅ | ❌ | ❌ | ❌ (Haskell) | ✅ | | Dive | ✅ | ❌ | ❌ | ❌ (Go) | ✅ | | SlimToolkit | ✅ | ✅* | ❌ | ❌ (Go) | ✅ | | dockerfile_lint | ✅ | ❌ | ❌ | ✅ | ❌ (2019) | | Dockershrink | ✅ | ✅ | ❌ | ✅ | ⚠️ (Beta) |

*SlimToolkit optimization can break apps with dynamic dependencies

Why dockerfile-slim?

• npm native - No binary installation, just npx
• Instant analysis - No Docker build required
• Actionable suggestions - Not just problems, but specific fixes
• Cost estimation - Justify optimization work to stakeholders
• Actively maintained - Unlike abandoned npm alternatives

CLI Options

Usage: dockerfile-slim [path] [options]

Arguments:
  path                    Path to Dockerfile or directory (default: ".")

Options:
  --json                  Output results as JSON
  --fix                   Generate an optimized Dockerfile
  -o, --output <file>     Output file for generated Dockerfile
  --dockerignore          Generate a .dockerignore file
  -V, --version           Output version number
  -h, --help              Display help

CI/CD Integration

# GitHub Actions example
- name: Analyze Dockerfile
  run: npx dockerfile-slim --json > dockerfile-analysis.json

- name: Check for critical issues
  run: |
    CRITICAL=$(cat dockerfile-analysis.json | jq '.optimizations | map(select(.severity == "critical")) | length')
    if [ "$CRITICAL" -gt 0 ]; then
      echo "Found $CRITICAL critical Dockerfile issues"
      exit 1
    fi

Optimization Database

Includes rules for:

Base Images:

• Ubuntu → Debian Slim → Alpine
• Node.js → node:slim → node:alpine → distroless
• Python → python:slim → python:alpine → distroless
• Go → golang:alpine → scratch/distroless
• Java → eclipse-temurin:jre → distroless

Common Issues:

• Missing multi-stage builds
• apt-get without cleanup
• npm/pip cache not cleaned
• Multiple RUN commands (layer bloat)
• Poor layer caching order
• Running as root
• Using :latest tag

Requirements

• Node.js 18.0.0 or higher

Contributing

Contributions welcome! Especially:

• Adding more optimization patterns
• Improving size estimates
• Adding more base image alternatives
• Better language-specific Dockerfile generation

Related Projects

• Hadolint - Dockerfile linter (Haskell)
• Dive - Image layer explorer (Go)
• SlimToolkit - Container optimizer (Go)

Support

This project is maintained in my free time. If it helped reduce your Docker image size or saved you debugging time, I'd really appreciate your support:

• ⭐ Star the repo—it helps others discover this tool
• 📢 Share with your team or on social media
• 🐛 Report bugs or suggest features
• ☕ Buy me a coffee if you'd like to support development

Thank you to everyone who has contributed, shared feedback, or helped spread the word!

License

MIT

Made with ❤️ for smaller Docker images