Docker Training

What is Docker?

Docker lets you package your app and everything it needs into a container.

Think of it like a lightweight box that runs your app the same way — on your laptop, on a server, or in the cloud.

Why Use Docker?

• Runs the same on your laptop, staging, and production
• Quick to start and stop — seconds, not minutes
• Easy to share with your team
• Keeps your host machine clean
• Uses less resources than a virtual machine

Installing Docker (Step 1)

We are using Ubuntu 25.04. Run this in your terminal:

curl https://get.docker.com | bash

Installing Docker (Step 2)

Add your user to the Docker group so you don't need sudo every time:

sudo usermod -aG docker $USER

Verify Your Installation

docker --version
docker run hello-world

If you see a welcome message from Docker — you're all set!

docker run — The Basics

docker run creates and starts a new container from an image.

• Image = the template (like a recipe)
• Container = the running app (like the cooked dish)

docker run nginx

This downloads the nginx image (if not local) and starts a container.

docker run — Useful Flags

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

# Give it a name
docker run -d --name my-web nginx

# Map a port (host:container)
docker run -d --name my-web -p 8080:80 nginx

# Set an environment variable
docker run -d -e MYSQL_ROOT_PASSWORD=secret mysql

• -d → run in the background
• --name → give the container a friendly name
• -p → port mapping: host port → container port
• -e → set an environment variable

docker run --rm — Temporary Container

# Run once, auto-remove when done
docker run --rm alpine echo "Hello from Docker!"

The --rm flag removes the container automatically when it stops. Perfect for one-time tasks — no leftover containers to clean up!

# Quick test with Python
docker run --rm python:3.12 python -c "print('Python says hi!')"

docker exec — Run Commands Inside

# Start a container first
docker run -d --name my-web nginx

# Run a command inside it
docker exec my-web cat /etc/os-release

# Open a shell inside the container
docker exec -it my-web bash

docker exec lets you run commands inside a running container.

What is Docker Compose?

When your app needs multiple containers (e.g. web server + database), Docker Compose lets you define and run them all with a single YAML file.

• One file to describe your whole setup
• One command to start everything
• Easy to share and version-control

docker compose up

# docker-compose.yml
services:
  web:
    image: nginx
    ports:
      - "8080:80"

# Start everything in the background
docker compose up -d

# Watch the logs
docker compose logs -f

# Start and rebuild if needed
docker compose up -d --build

The -d flag runs in the background. Without it, you see logs in your terminal.

docker compose down & pull

# Stop and remove containers
docker compose down

# Stop and remove everything including volumes
docker compose down --volumes

# Pull latest images (no start)
docker compose pull

# Common workflow: pull then restart
docker compose pull && docker compose up -d

down stops and removes containers but keeps your data by default. pull grabs the latest images without starting anything.

Volumes — Keeping Your Data

Containers are temporary. When removed, their files are gone.

Volumes save data permanently outside the container.

Two main types:

• Bind mounts — link a folder on your machine to the container
• Named volumes — let Docker manage where data is stored

Bind Mount (./shared-drive)

services:
  web:
    image: nginx
    volumes:
      - ./html:/usr/share/nginx/html

Links a folder on your host machine to a folder inside the container.

Named Volumes (Docker Managed)

services:
  db:
    image: mysql
    volumes:
      - db-data:/var/lib/mysql

volumes:
  db-data:

Docker decides where to store the data on your host. Best for data you want to keep but don't need to edit directly (like database files).

Port Mapping

services:
  web:
    image: nginx
    ports:
      - "8080:80"     # HOST:CONTAINER
      - "443:443"
  db:
    image: mysql
    ports:
      - "3306:3306"

The format is HOST_PORT : CONTAINER_PORT

• Left side → what you type in your browser
• Right side → what the app uses inside the container

docker ps — See What's Running

# Show running containers
docker ps

# Show ALL containers (including stopped)
docker ps -a

# Show just the container IDs
docker ps -q

docker ps shows running containers, their names, ports, and status.

Cleaning Up — docker system prune

Over time Docker collects stopped containers, unused images, and junk.

# Remove unused data (prompts for confirmation)
docker system prune

# Remove EVERYTHING unused, no confirmation
docker system prune -af

# Also remove volumes
docker system prune -af --volumes

• -a → remove ALL unused images
• -f → skip the "are you sure?" prompt

Quick Cleanup Commands

# Remove a specific container
docker rm my-container

# Remove a specific image
docker rmi nginx

# Stop all running containers
docker stop $(docker ps -q)

# Remove all stopped containers
docker container prune

Cheat Sheet

Happy Dockering!

Practice makes perfect.
Try running different images, break things, and learn from it.

Docker is your friend!

Thank you!