Docker Images

5. Docker Images

🖼️ What is a Docker Image?

A Docker image is a read-only template used to create containers. Think of it like a recipe:

The recipe (image) tells you exactly what ingredients and steps are needed
Every time you cook (run), you get the same dish (container)
Many people can use the same recipe to cook the same dish

Images are built in layers — each layer represents a set of file system changes.

📚 Image Layers

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  <!-- Background -->
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  <text x="250" y="28" text-anchor="middle" font-size="14" font-weight="bold" fill="#222">Docker Image Layer System</text>

  <!-- Layer 5 - App Code (top, writable) -->
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  <text x="250" y="69" text-anchor="middle" font-size="12" font-weight="bold" fill="#1b5e20">Your App Code Layer</text>
  <text x="250" y="85" text-anchor="middle" font-size="10" fill="#2e7d32">app.js, package.json, etc.</text>
  <text x="420" y="78" text-anchor="end" font-size="9" fill="#555">Writable (Container)</text>

  <!-- Arrow -->
  <text x="250" y="108" text-anchor="middle" font-size="16" fill="#999">▲ built on top of</text>

  <!-- Layer 4 - npm packages -->
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  <text x="250" y="132" text-anchor="middle" font-size="12" fill="#1b5e20">npm dependencies Layer</text>
  <text x="250" y="147" text-anchor="middle" font-size="10" fill="#388e3c">node_modules/</text>

  <!-- Layer 3 - Node.js -->
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  <text x="250" y="179" text-anchor="middle" font-size="12" fill="#0d47a1">Node.js Runtime Layer</text>
  <text x="250" y="194" text-anchor="middle" font-size="10" fill="#1565c0">node, npm binaries</text>

  <!-- Layer 2 - OS utilities -->
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  <text x="250" y="226" text-anchor="middle" font-size="12" fill="#4a148c">OS Utilities Layer</text>
  <text x="250" y="241" text-anchor="middle" font-size="10" fill="#6a1b9a">curl, bash, apt packages</text>

  <!-- Layer 1 - Base OS -->
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  <text x="250" y="273" text-anchor="middle" font-size="12" fill="#bf360c">Base OS Layer</text>
  <text x="250" y="288" text-anchor="middle" font-size="10" fill="#e65100">ubuntu:22.04 (slim)</text>

  <!-- Read-only label -->
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  <text x="60" y="195" text-anchor="middle" font-size="10" fill="#f57f17" transform="rotate(-90, 60, 195)">READ-ONLY (Image)</text>

  <!-- Bottom note -->
  <text x="250" y="330" text-anchor="middle" font-size="10" fill="#777">Each layer is cached — unchanged layers are reused across builds</text>
</svg>

Why layers matter:

Layers are cached — if a layer doesn’t change, Docker reuses it
Speeds up builds dramatically
Multiple images can share identical base layers, saving disk space

📋 Key Image Commands

# Pull an image from Docker Hub
docker pull nginx
docker pull node:18
docker pull ubuntu:22.04

# List all local images
docker images
# or
docker image ls

# Remove an image
docker rmi nginx
docker image rm nginx

# Remove all unused images
docker image prune

# View image layer history
docker history node:18

# Inspect image details
docker inspect nginx

# Search for images on Docker Hub
docker search nginx

# Pull a specific version (tag)
docker pull node:18-alpine
docker pull postgres:15.2

🏷️ Understanding Image Tags

# Format: repository:tag
nginx           # same as nginx:latest
nginx:latest    # latest stable version
nginx:1.25      # specific version
node:18         # Node.js 18.x
node:18-alpine  # Node.js 18 on Alpine Linux (tiny!)
node:18-slim    # Node.js 18, reduced size

💡 Tip: Always use specific version tags in production (node:18.17.0, not node:latest) to avoid unexpected updates breaking your app.

🌟 Official vs Community Images

Type	Example	Description
Official	`nginx`, `node`, `postgres`	Maintained by Docker/vendor
Verified Publisher	`bitnami/nginx`	Trusted companies
Community	`username/myapp`	Individual users

⚠️ Common Mistakes with Images

Using latest tag in production — always use specific versions
Pulling huge images when Alpine variants exist (e.g., node:18-alpine is ~50MB vs node:18 at ~1GB)
Forgetting to docker image prune — unused images pile up fast