How to Get Started with Docker for Beginners

What is Docker

Docker is a platform that enables developers to package applications and their dependencies into containers. Containers are lightweight, portable, and consistent across different environments, making development, testing, and deployment more efficient.

Docker Concepts and Terminology

Installing Docker

Windows and macOS:

1. Download Docker Desktop from Docker’s website.
2. Run the installer and follow the prompts.
3. After installation, launch Docker Desktop.

Linux (Ubuntu example):

sudo apt-get update
sudo apt-get install     ca-certificates     curl     gnupg

sudo install -m 0755 -d /etc/apt/keyrings
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/keyrings/docker.gpg

echo   "deb [arch="$(dpkg --print-architecture)" signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu   $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

sudo apt-get update
sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin

To verify installation:

docker --version

Running Your First Container

Start a simple container using an official image:

docker run hello-world

This downloads the hello-world image and runs it in a container, printing a test message.

Docker Images and Containers

• Listing Images:
  bash
docker images
• Listing Running Containers:
  bash
docker ps
• Listing All Containers (including stopped):
  bash
docker ps -a
• Stopping and Removing Containers:
  bash
docker stop <container_id>
docker rm <container_id>
• Removing Images:
  bash
docker rmi <image_id>

Building a Docker Image

Create a simple Dockerfile for a Python application:

Directory Structure:

myapp/
  ├─ app.py
  └─ Dockerfile

app.py:

print("Hello from Docker!")

Dockerfile:

FROM python:3.10-slim
COPY app.py .
CMD ["python", "app.py"]

Building the Image:

docker build -t my-python-app .

Running the Container:

docker run my-python-app

Working with Docker Hub

• Login:
  bash
docker login
• Tagging an Image:
  bash
docker tag my-python-app username/my-python-app:latest
• Pushing to Docker Hub:
  bash
docker push username/my-python-app:latest
• Pulling an Image:
  bash
docker pull username/my-python-app:latest

Docker Volumes

Docker volumes persist data beyond the container lifecycle.

• Create and Mount a Volume:
  bash
docker volume create mydata
docker run -d -v mydata:/data busybox
• List Volumes:
  bash
docker volume ls

Networking in Docker

• Bridge Network (default): Containers can communicate on the same host.
• Custom Network:
  bash
docker network create mynetwork
docker run -d --name app1 --network mynetwork busybox sleep 3600
docker run -d --name app2 --network mynetwork busybox sleep 3600
docker exec -it app1 ping app2

Common Docker Commands Summary

Best Practices for Beginners

• Use official images as base images where possible.
• Keep images lean by removing unnecessary files in Dockerfile.
• Use .dockerignore to exclude files from builds.
• Avoid running applications as root inside containers.
• Tag images with meaningful version numbers.

Example: Running a Web Application

Dockerfile for Flask App:

FROM python:3.10-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

requirements.txt:

flask

app.py:

from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello():
    return "Hello from Docker Flask!"
if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

Build and Run:

docker build -t flask-app .
docker run -p 5000:5000 flask-app

Access the app at http://localhost:5000.

Docker Compose Basics

Docker Compose manages multi-container applications using a docker-compose.yml file.

Sample docker-compose.yml:

version: '3'
services:
  web:
    build: .
    ports:
      - "5000:5000"

Usage:

docker compose up

This command builds and starts all services defined in the file.

Troubleshooting Tips

• Use docker logs <container> to view container output.
• Remove unused resources with docker system prune.
• Restart Docker Desktop if containers fail to start due to resource errors.
• Use docker inspect for detailed container and image information.