Images

What are Docker Images?¶

A Docker image is a read-only template that contains: * Application code * Runtime environment * System tools and libraries * Configuration files

Images are the building blocks of containers. When you run an image, it becomes a running container.

Image Layers¶

Docker images are built in layers. Each layer represents an instruction in the Dockerfile:

Layer 5: Copy application code
Layer 4: Install app dependencies
Layer 3: Install Node.js
Layer 2: Install system packages
Layer 1: Base OS (e.g., Ubuntu)

Benefits of layers: * Caching - Unchanged layers are reused, speeding up builds * Efficiency - Multiple images can share common layers * Version control - Each layer is versioned

Working with Images¶

Listing Images¶

# List all images on your system
docker images

# Alternative command
docker image ls

Output shows: * Repository name * Tag (version) * Image ID * Size * Creation date

Pulling Images¶

# Pull an image from Docker Hub
docker pull nginx

# Pull a specific version
docker pull nginx:1.21

# Pull from a specific registry
docker pull gcr.io/my-project/my-image

Searching for Images¶

# Search Docker Hub for images
docker search nginx

# Limit results
docker search --limit 5 nginx

Removing Images¶

# Remove an image by name
docker rmi nginx

# Remove an image by ID
docker rmi abc123def456

# Remove unused images
docker image prune

# Remove all unused images
docker image prune -a

# Force remove an image
docker rmi -f nginx

Image Tags¶

Tags identify specific versions of an image:

# Format: repository:tag
nginx:latest       # Latest stable version
nginx:1.21         # Specific version
nginx:1.21-alpine  # Alpine Linux variant

Common conventions: * latest - Most recent stable version * alpine - Minimal Alpine Linux-based version * Version numbers - Specific releases (e.g., 3.9, 14.17.0)

Inspecting Images¶

# View detailed information about an image
docker inspect nginx

# View image history (layers)
docker history nginx

Image Naming¶

Full image name format:

registry.domain.com:port/namespace/repository:tag

Examples:

nginx:latest                           # Docker Hub default
docker.io/library/nginx:latest         # Full Docker Hub path
gcr.io/project-id/my-app:v1.0         # Google Container Registry
myregistry.com:5000/team/app:latest    # Private registry

Best Practices¶

Use official images - Start with official base images from Docker Hub
Use specific tags - Avoid latest in production
Keep images small - Use Alpine variants when possible
Regular updates - Keep base images updated for security
Clean up - Remove unused images to save disk space

Skill++¶

The largest Docker images can be several gigabytes, while minimal Alpine-based images can be just 5MB!

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