🔹 Introduction to Docker

If you’ve ever faced the dreaded “It works on my machine but not on yours” problem — Docker is the hero you need.

Docker allows developers to package applications into isolated environments called containers, ensuring they run the same everywhere.

✅ Think of it like this:
You built a school project website that runs fine on your laptop.
But when you copy it to your friend’s system, it crashes because they have a different PHP version.

With Docker, you can package:

• Your code
• PHP
• All dependencies

into a single container.
Now it works everywhere 🚀.

⚙️ Docker Architecture

Docker has two main parts:

🖥️ 1. Docker Client

The tool we use to interact with Docker.

Runs commands like:

• docker run → Start a container
• docker pull → Download an image

Can also communicate with a remote Docker Daemon over a network.

⚡ 2. Docker Daemon

The engine of Docker running in the background.

Responsible for:

• Building images
• Running containers
• Managing containers, images, networks, and volumes

🔗 Communication

Client ↔ Daemon communicate via:

• REST API
• Unix socket
• Network interface

👉 Analogy:
You (client) say “Make tea!” ☕ and the kitchen (daemon) makes it.

🧩 Docker Components

📦 1. Docker Images

• Read-only templates used to create containers
• Built in layers

If one layer changes, only that part rebuilds.

👉 Example:

• Base: ubuntu
• Add Python → new layer
• Add Flask → another layer

docker pull ubuntu

docker pull ubuntu

🏃 2. Docker Containers

• A running instance of an image
• Can be started, stopped, deleted, or moved
• Has its own read-write layer
• Connects to storage and networks

By default, containers are ephemeral (data is lost when removed) unless a volume is attached.

📌 Flow:

1. Pull image
2. Create container
3. Add read-write layer
4. Connect to virtual network
5. Get unique container IP
6. Interact via terminal

docker container create --name shan ubuntu /bin/bash

docker container create --name shan ubuntu /bin/bash

🔥 3. Docker Engine

The core runtime that builds and runs containers locally.

☁️ 4. Docker Hub

A cloud-based repository to store and share Docker images.

📝 5. Dockerfile

A script with step-by-step instructions to build an image.
Executed top to bottom by the Docker Daemon.

📌 Workflow:
Dockerfile → docker build → Docker Image → docker run → Container

✅ Example:

FROM ubuntu
RUN apt-get update && apt-get install -y nginx
CMD ["nginx", "-g", "daemon off;"]

FROM ubuntu
RUN apt-get update && apt-get install -y nginx
CMD ["nginx", "-g", "daemon off;"]

📦 Docker Registry

A service that stores Docker images.
Docker Hub is the most popular public registry.

🔧 Common Docker Commands

🔹 Image Commands

• docker image ls → List images
• docker pull <image> → Download image
• docker rmi <image> → Remove image
• docker history <image> → View layers

🔹 Container Commands

• docker run <image> → Run container
• docker ps -a → List containers
• docker exec -it <id> /bin/bash → Open terminal inside container
• docker logs <id> → View logs

👉 Example: Run Nginx on port 80

docker run -d -p 80:80 nginx

docker run -d -p 80:80 nginx

🥊 Docker vs Virtual Machine

👉 Example:

• Want to run 10 microservices? → Use containers
• Want to run Windows on Linux? → Use a VM