Introduction to Docker

What is Docker?

Docker is a platform for building, shipping, and running applications in containers. Containers package an application with all its dependencies, ensuring it runs consistently across any environment.

Why Docker for DevOps?

Consistency — "Works on my machine" is solved
Isolation — Applications don't interfere with each other
Portability — Run anywhere Docker is installed
Efficiency — Lighter than virtual machines
Scalability — Easy to scale with orchestrators

Containers vs Virtual Machines

┌─────────────────────────────────┐
│        Virtual Machines          │
├────────┬────────┬────────┬──────┤
│  App A │  App B │  App C │      │
│  Bins  │  Bins  │  Bins  │      │
│Guest OS│Guest OS│Guest OS│      │
├────────┴────────┴────────┤      │
│       Hypervisor          │      │
├───────────────────────────┤      │
│        Host OS            │      │
└───────────────────────────┘      │

┌─────────────────────────────────┐
│          Containers              │
├────────┬────────┬────────┬──────┤
│  App A │  App B │  App C │      │
│  Bins  │  Bins  │  Bins  │      │
├────────┴────────┴────────┤      │
│      Docker Engine        │      │
├───────────────────────────┤      │
│        Host OS            │      │
└───────────────────────────┘

Containers share the host OS kernel, making them much lighter and faster to start.

Installing Docker

# Ubuntu
sudo apt update
sudo apt install docker.io -y
 
# Start and enable Docker
sudo systemctl start docker
sudo systemctl enable docker
 
# Add your user to the docker group
sudo usermod -aG docker $USER
 
# Log out and back in, then verify
docker --version

Your First Container

# Run a hello-world container
docker run hello-world
 
# Run an interactive Ubuntu container
docker run -it ubuntu bash
 
# Inside the container:
cat /etc/os-release
exit

Docker Architecture

Component	Role
Docker Client	CLI tool (`docker` command)
Docker Daemon	Background service managing containers
Images	Read-only templates for containers
Containers	Running instances of images
Registry	Storage for Docker images (Docker Hub)

Essential Commands

# List running containers
docker ps
 
# List all containers (including stopped)
docker ps -a
 
# List downloaded images
docker images
 
# Pull an image
docker pull nginx
 
# Run a container in background
docker run -d -p 8080:80 nginx
 
# Stop a container
docker stop <container_id>
 
# Remove a container
docker rm <container_id>

Summary

You've learned:

What containers are and how they differ from VMs
How to install Docker
How to run your first container
Docker's architecture and essential commands

Next Steps

Next, we'll learn about Dockerfiles — how to build your own custom container images.

Learning Objectives