Docker Images - Docker & DevOps Tutorial | TechLead

What Are Docker Images?

A Docker image is a read-only template that contains the application code, runtime, libraries, environment variables, and configuration files needed to run a container. Images are built in layers, with each layer representing a set of filesystem changes.

Image Layers

Each instruction in a Dockerfile creates a new layer. Layers are cached and reused, making builds faster and images more efficient.

Layer 4: COPY app code

Layer 3: RUN npm install

Layer 2: COPY package.json

Layer 1: FROM node:20-alpine (Base Image)

Working with Images

Pulling Images

# Pull an image from Docker Hub
docker pull nginx
docker pull node:20-alpine
docker pull postgres:16

# Pull a specific version
docker pull nginx:1.25

# Pull from a different registry
docker pull ghcr.io/owner/image:tag

Listing Images

# List all local images
docker images

# Filter images
docker images --filter "reference=node*"

# Show image sizes
docker images --format "table {{.Repository}}\t{{.Tag}}\t{{.Size}}"

Inspecting Images

# Get detailed image information
docker inspect nginx

# See image history (layers)
docker history nginx

# Show image digest
docker images --digests

Image Tags

Tags are labels that point to specific image versions. The latest tag is the default but using specific version tags in production is a best practice.

# Common tag patterns
node:20           # Major version
node:20.11        # Minor version
node:20-alpine    # Alpine Linux variant (smaller)
node:20-slim      # Slim variant
node:latest       # Latest version (avoid in production)

# Tag an existing image
docker tag myapp:latest myapp:v1.0.0

# Tag for pushing to a registry
docker tag myapp:latest username/myapp:v1.0.0

Managing Images

# Remove an image
docker rmi nginx

# Remove all unused images
docker image prune

# Remove ALL images (careful!)
docker image prune -a

# Save an image to a tar archive
docker save -o myapp.tar myapp:latest

# Load an image from a tar archive
docker load -i myapp.tar

Docker Hub

Docker Hub is the default public registry for Docker images. It hosts official images (like nginx, node, postgres) and community-created images.

# Login to Docker Hub
docker login

# Search for images
docker search nginx

# Push an image to Docker Hub
docker push username/myapp:v1.0.0

# Logout
docker logout

Best Practices

🏷️ Always use specific version tags in production (never latest)
📦 Use Alpine-based images for smaller image sizes
🧹 Regularly clean up unused images with docker image prune
🔒 Only pull images from trusted sources and official repositories