Images & Containers

Docker Core Concepts

Understanding images and containers is fundamental to working with Docker.

Images vs Containers

Docker Image Basics

# Pull an image from Docker Hub
docker pull nginx
docker pull nginx:1.25-alpine

# List local images
docker images
docker image ls

# Image details
docker inspect nginx

# Remove an image
docker rmi nginx
docker image rm nginx:1.25-alpine

# Remove unused images
docker image prune
docker image prune -a  # Remove all unused

Image Layers

Docker images are built in layers. Each instruction in a Dockerfile creates a new layer.

Image Layer Structure:
+-------------------+
| Application Code  |  <- Your code (changes frequently)
+-------------------+
| Dependencies      |  <- npm install, pip install
+-------------------+
| Runtime           |  <- Node.js, Python
+-------------------+
| Base OS           |  <- Alpine, Debian, Ubuntu
+-------------------+

# View image layers
docker history nginx

# Show layer sizes
docker history --no-trunc nginx

Container Basics

# Create and start a container
docker run nginx

# Run in detached mode (background)
docker run -d nginx

# Run with name
docker run -d --name my-nginx nginx

# Run with port mapping
docker run -d -p 8080:80 nginx

# Run with environment variables
docker run -d -e MYSQL_ROOT_PASSWORD=secret mysql

# Run interactively
docker run -it ubuntu bash

Container Lifecycle

# List running containers
docker ps

# List all containers (including stopped)
docker ps -a

# Stop a container
docker stop my-nginx

# Start a stopped container
docker start my-nginx

# Restart a container
docker restart my-nginx

# Remove a container
docker rm my-nginx

# Remove running container (force)
docker rm -f my-nginx

# Remove all stopped containers
docker container prune

Container Interaction

# Execute command in running container
docker exec -it my-nginx bash

# View container logs
docker logs my-nginx
docker logs -f my-nginx      # Follow logs
docker logs --tail 100 my-nginx

# View container processes
docker top my-nginx

# View resource usage
docker stats
docker stats my-nginx

# Copy files to/from container
docker cp ./file.txt my-nginx:/app/
docker cp my-nginx:/app/file.txt ./

Container Inspection

# Full container details
docker inspect my-nginx

# Get specific field
docker inspect --format='{{.NetworkSettings.IPAddress}}' my-nginx

# View port mappings
docker port my-nginx

# View filesystem changes
docker diff my-nginx

Container Best Practices

1. One process per container - Run single service per container
2. Immutable containers - Don't modify running containers
3. Use specific tags - Avoid latest in production
4. Clean up regularly - Prune unused resources
5. Use restart policies - --restart unless-stopped