Author: Shams Raza
Tags: #Docker #DevOps #Containers #Linux #Troubleshooting
Reading Time: ~7 min

GitHub: Docker-Learn

Containers are now a standard in modern application development, and Docker is the most popular containerization platform. Whether you're a beginner or looking to level up your Docker skills, this guide walks you through:

✅ Docker basics
✅ Useful commands (Beginner → Advanced)
✅ Common Docker issues and how to fix them

🐳 What is Docker?

Docker is an open-source platform to build, ship, and run applications inside containers. It packages software with all dependencies, ensuring consistent environments across development, testing, and production.

🧱 Docker Basics

📦 1. Install Docker

• Windows/Mac: Install Docker Desktop from https://www.docker.com/products/docker-desktop

• Linux:

sudo apt update
sudo apt install docker.io -y
sudo systemctl enable --now docker

Add your user to the Docker group:

sudo usermod -aG docker $USER

🛠️ 2. Basic Docker Commands

Example:

docker run -it ubuntu bash

⚙️ Intermediate Docker Commands

🧰 Image Building

Create a Dockerfile:

FROM python:3.11-slim
WORKDIR /app
COPY . .
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

Build and tag the image:

docker build -t my-python-app .
docker images

🧪 Docker Volumes

docker volume create my_data
docker run -v my_data:/data busybox

Useful for persisting data across container restarts.

🧑‍🤝‍🧑 Docker Networking

docker network create my_net
docker run -d --name app1 --network my_net nginx
docker run -it --network my_net busybox

Now from busybox you can ping app1.

🧠 Advanced Docker Commands

🧭 Multi-Stage Builds

Efficiently build lean images:

# stage 1
FROM golang:1.19 as builder
WORKDIR /src
COPY . .
RUN go build -o app .

# stage 2
FROM alpine
COPY --from=builder /src/app /app
CMD ["/app"]

🧪 Health Checks

HEALTHCHECK CMD curl --fail http://localhost:80 || exit 1

Add this in Dockerfile to check container health.

🐳 Docker Compose (for multi-container apps)

docker-compose.yml:

version: "3.9"
services:
  web:
    image: nginx
    ports:
      - "80:80"
  app:
    build: .
    volumes:
      - .:/app
    depends_on:
      - web

Run with:

docker-compose up --build

🧯 Docker Troubleshooting

❗ Error: "Permission denied" when using Docker

Fix:

sudo usermod -aG docker $USER
newgrp docker

❗ Container exits immediately

Check logs:

docker logs <container_id>

Run in interactive mode to debug:

docker run -it <image> /bin/bash

❗ Port already in use

Check and kill:

sudo lsof -i :80
sudo kill -9 <PID>

❗ Cannot connect to Docker daemon

Check service:

sudo systemctl status docker
sudo systemctl restart docker

❗ Image too large

Use Alpine base:

FROM python:3.11-alpine

Clean up:

docker system prune
docker builder prune

🔚 Conclusion

Docker makes development and deployment faster, more consistent, and scalable. Mastering its commands and troubleshooting skills helps you become a better DevOps engineer or backend developer.

✅ Start small, build fast, and troubleshoot confidently.

📚 Bonus Resources

• Docker Docs

• Play with Docker

• Awesome Docker GitHub

💬 Have questions or tips to share? Drop them in the comments below!
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