Docker Images: The Art of Choosing the Right Building Blocks 🏗️

The Story of the Perfect Backpack

Imagine you’re packing a backpack for a trip. You could throw in everything you own—but then it would be heavy, slow, and hard to carry. Instead, you pick only what you need. A light backpack means you travel faster and easier!

Docker images are exactly like backpacks. They hold everything your app needs to run. A well-packed image is small, fast, and secure. A bloated one? Slow to download, wastes space, and has more things that could break.

Let’s learn how to pack the perfect Docker backpack!

1. Image Inspection and History: Peeking Inside the Backpack

What’s Inside?

Before you buy a backpack, you’d check what’s already inside, right? Docker lets you do the same with images!

docker inspect shows you everything about an image—like reading the ingredient label on food.

docker inspect nginx:latest

This tells you:

• How big is it?
• What commands does it run?
• What ports does it use?
• Who made it?

Who Packed It? (Image History)

docker history shows you every step that built the image—like a recipe!

docker history nginx:latest

Example output:

IMAGE         CREATED BY                 SIZE
abc123        CMD ["nginx", "-g"...]     0B
def456        COPY nginx.conf /etc...    2.3KB
ghi789        RUN apt-get install...     50MB
jkl012        FROM debian:bullseye       80MB

Each line is a layer—like layers in a cake. More on this soon!

Why This Matters

• Find bloat: Spot which step added 500MB!
• Debug problems: See exactly what was installed
• Trust check: Verify the image does what you expect

2. Image Size Optimization: Traveling Light

The Problem with Big Images

Big images mean:

• ⏳ Slow downloads (imagine waiting 10 minutes to start!)
• 💸 More storage costs
• 🐌 Slower deployments
• 🎯 More attack surface (more stuff = more vulnerabilities)

Quick Wins for Smaller Images

1. Start with a smaller base (we’ll cover this soon!)

2. Clean up after yourself

RUN apt-get update && \
    apt-get install -y curl && \
    rm -rf /var/lib/apt/lists/*

3. Use multi-stage builds

# Stage 1: Build
FROM node:18 AS builder
COPY . .
RUN npm run build

# Stage 2: Run (much smaller!)
FROM node:18-alpine
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/index.js"]

The builder stage has all the tools. The final image only has what you need to run!

Real Size Comparison

3. Layer Caching: The Magic of Not Repeating Yourself

How Layers Work

Think of layers like floors in a building:

graph TD
    A["🏠 Base Image<br>#40;Floor 1#41;"] --> B["📦 Install Dependencies<br>#40;Floor 2#41;"]
    B --> C["📁 Copy Source Code<br>#40;Floor 3#41;"]
    C --> D["🔨 Build App<br>#40;Floor 4#41;"]
    D --> E["🚀 Final Image"]

If Floor 1 changes, ALL floors above must be rebuilt. But if only Floor 4 changes? Only that floor is rebuilt!

The Golden Rule

Put things that change LEAST at the TOP, things that change MOST at the BOTTOM.

❌ Bad Order (slow builds):

FROM node:18
COPY . .
RUN npm install

Every code change = reinstall ALL packages!

✅ Good Order (fast builds):

FROM node:18
COPY package*.json ./
RUN npm install
COPY . .

Package files rarely change, so npm install is cached!

Cache-Friendly Tips

1. Copy dependency files first (package.json, requirements.txt)
2. Install dependencies before copying code
3. Combine RUN commands to reduce layers
4. Use .dockerignore to skip unnecessary files

4. Choosing Base Images: Picking Your Foundation

The Foundation Matters

Your base image is like choosing between a mansion and a tiny house. Both can shelter you, but one is way more than you need!

Decision Flowchart

graph TD
    A["🤔 What do I need?"] --> B{"Need a shell<br>and tools?"}
    B -->|Yes| C{"How minimal?"}
    B -->|No| D["🔥 Scratch or&lt;br&gt;Distroless"]
    C -->|Standard| E["📦 Debian/Ubuntu"]
    C -->|Minimal| F["🏔️ Alpine"]

Base Image Options

5. Official and Verified Images: Trust Matters

What Makes an Image “Official”?

Official images are like products with a quality seal. Docker and the software makers maintain them!

Look for the blue “Official Image” badge on Docker Hub.

# Official nginx image
docker pull nginx

# NOT official (random user)
docker pull someuser123/nginx

Verified Publishers

Some companies are Verified Publishers—Docker checked that they are who they claim to be.

Why Use Official Images?

• ✅ Regularly updated with security patches
• ✅ Best practices baked in
• ✅ Documentation is solid
• ✅ Trusted source (no hidden malware)

Example: Finding Official Images

# Search Docker Hub for official images
docker search --filter is-official=true nginx

6. Alpine Images: The Lightweight Champion

What is Alpine?

Alpine Linux is a super tiny Linux distribution. While Ubuntu is ~80MB, Alpine is just ~5MB!

Think of it like: A full suitcase (Ubuntu) vs. a small pouch (Alpine).

Using Alpine

Most official images have Alpine versions:

# Regular Node.js (~400MB)
docker pull node:18

# Alpine Node.js (~50MB)
docker pull node:18-alpine

Alpine Gotchas

Alpine uses musl instead of glibc. Some programs might not work!

# If something breaks on Alpine,
# you might need to install gcompat
RUN apk add --no-cache gcompat

When to Use Alpine

7. Scratch and Distroless: The Minimalists

Scratch: Starting from Zero

Scratch is literally an empty image. Nothing at all!

FROM scratch
COPY myapp /myapp
CMD ["/myapp"]

Perfect for:

• Go binaries (they’re self-contained!)
• Rust binaries
• Any statically compiled program

Size: Whatever your binary is. Nothing extra!

Distroless: Just Enough

Google’s Distroless images have a runtime but NO:

• Shell
• Package manager
• Any extra tools

FROM gcr.io/distroless/static-debian12
COPY myapp /myapp
CMD ["/myapp"]

Comparison Table

Security Bonus

Fewer things = fewer vulnerabilities!

If there’s no shell, attackers can’t get a shell. If there’s no curl, they can’t download tools. Simple!

The Complete Journey

graph TD
    A["🔍 Inspect Images"] --> B["📏 Optimize Size"]
    B --> C["⚡ Use Layer Caching"]
    C --> D["🏠 Choose Base Image"]
    D --> E{"What type?"}
    E --> F["🏔️ Alpine<br>#40;Small + Shell#41;"]
    E --> G["🔒 Distroless<br>#40;Secure#41;"]
    E --> H["⚡ Scratch<br>#40;Minimal#41;"]
    E --> I["📦 Official<br>#40;Standard#41;"]

Quick Summary

1. Inspect images to understand what’s inside
2. Optimize by removing unnecessary files and using multi-stage builds
3. Cache layers by ordering Dockerfile commands smartly
4. Choose the right base for your needs
5. Trust official images from verified sources
6. Go Alpine for small images with shell access
7. Use Scratch/Distroless for maximum security and minimum size

Remember This!

“A Docker image is like a backpack. Pack only what you need, and you’ll travel fast and light!” 🎒

You’re now ready to build lean, secure Docker images like a pro!