Day 1 - Build a Minimal Docker Image

Description

You're tasked with containerizing a small application, but your manager is concerned about image size and security. Large images slow down deployments, cost more to store, and have larger attack surfaces. Your goal is to create the smallest, most efficient Docker image possible.

Task

Create the smallest working Docker image you can for a tiny Go application.

Requirements:

• Image must be under 25MB
• Application must run successfully
• Use multi-stage builds
• Use Alpine Linux base images or scratch
• Compile Go binary statically for minimal final image

Target

• Image Size: Under 25MB (under 10MB possible with scratch!)
• Startup Time: Under 1 second
• Security: Minimal attack surface with Alpine/scratch base

Sample App

Go Example (main.go)

package main

import (
	"fmt"
	"net/http"
)

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "text/plain")
		w.WriteHeader(http.StatusOK)
		fmt.Fprint(w, "Hello from Advent of DevOps!")
	})

	fmt.Println("Server running on port 8000...")
	if err := http.ListenAndServe(":8000", nil); err != nil {
		fmt.Printf("Error starting server: %v\n", err)
	}
}

Result

You should achieve:

• ✅ Docker image under 25MB (15MB Alpine, 7MB scratch)
• ✅ Functional Go web server responding on port 8000
• ✅ Container runs as non-root user (Alpine version)
• ✅ Fast build and startup times
• ✅ Static binary with zero runtime dependencies

Validation

# Check image size
docker images advent-day1 --format "table {{.Repository}}\t{{.Tag}}\t{{.Size}}"
# Alpine version: ~15MB
# Scratch version: ~7MB

# Verify non-root user (Alpine version only)
docker run --rm advent-day1:alpine whoami
# Should show: appuser

# Test application
docker run -d -p 8000:8000 --name test advent-day1:alpine
curl http://localhost:8000
# Should return: Hello from Advent of DevOps!
docker stop test && docker rm test

# Inspect image layers
docker history advent-day1:alpine
docker history advent-day1:scratch

Bonus: Python and Node.js Alternatives

If you prefer Python or Node.js, here are realistic minimal images. Note that the 25MB target is not achievable with interpreted languages due to runtime requirements.

Python (Realistic: ~60MB)

FROM python:3.12-alpine

RUN addgroup -g 1001 appgroup && \
    adduser -D -u 1001 -G appgroup appuser

WORKDIR /app
COPY app.py .

USER appuser
EXPOSE 8000

CMD ["python", "app.py"]

Python app.py:

from http.server import HTTPServer, BaseHTTPRequestHandler

class SimpleHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header('Content-type', 'text/plain')
        self.end_headers()
        self.wfile.write(b'Hello from Advent of DevOps!')

if __name__ == '__main__':
    server = HTTPServer(('0.0.0.0', 8000), SimpleHandler)
    print('Server running on port 8000...')
    server.serve_forever()

Note: Python Alpine images are ~50-60MB minimum due to the Python interpreter and standard library.

Node.js (Realistic: ~70MB)

FROM node:20-alpine

RUN addgroup -g 1001 appgroup && \
    adduser -D -u 1001 -G appgroup appuser

WORKDIR /app
COPY app.js .

USER appuser
EXPOSE 8000

CMD ["node", "app.js"]

Node.js app.js:

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello from Advent of DevOps!');
});

server.listen(8000, '0.0.0.0', () => {
  console.log('Server running on port 8000...');
});

Note: Node.js Alpine images are ~60-70MB minimum due to the Node.js runtime and V8 engine.

Links

• Docker Multi-Stage Builds
• Alpine Linux Docker Images
• Docker Best Practices
• Go Docker Official Images
• Building Minimal Go Docker Images
• Scratch Base Images

Share Your Success

Completed this challenge? Share your achievement!

Tag @thedevopsdaily on X (formerly Twitter) with:

• Your final image size
• Which approach you used (Alpine or scratch)
• Any additional optimizations you discovered
• Screenshots of your running container

Use hashtags: #AdventOfDevOps #Docker #Day1 #GoLang