🐳 Docker – Full Beginner Tutorial (Concepts + Example Commands)¶

Goal: You should understand what happens, not just what to type.

1️⃣ Why Docker Exists (With Example)¶

Concept¶

Applications fail across machines because:

OS versions differ
Library versions differ
Runtime versions differ

Docker packages the environment with the app.

Example Command (just for idea)¶

docker run node:18

📌 Meaning:

You instantly get Node.js 18
No installation on your system
Same environment everywhere

2️⃣ What Docker Actually Runs¶

Concept¶

Docker runs:

A process
In an isolated environment
Using the host OS kernel

It does NOT run a full OS.

Example¶

docker run ubuntu

What actually happens:

Ubuntu filesystem is loaded
Ubuntu kernel is NOT loaded
Host Linux kernel is reused

3️⃣ Docker Architecture (Client → Engine)¶

Concept Flow¶

You → Docker CLI → Docker Engine → Container

You talk to Docker Docker Engine does the work

Example¶

docker ps

Meaning:

CLI asks Engine:

“Which containers are running?”

4️⃣ Docker Images (Blueprint)¶

Concept¶

An image is:

Read-only
Layered
Reusable

Image ≠ Running app Image = Template

Example¶

docker images

Shows:

All blueprints available locally

5️⃣ Docker Containers (Running Instance)¶

Concept¶

A container is:

A running image
Has its own filesystem & network
Is temporary by default

Example¶

docker run nginx

Meaning:

Image → Container
App starts running

6️⃣ Image Layers (Why Docker Is Efficient)¶

Concept¶

Images are built in layers:

Layer 1 → OS base
Layer 2 → Runtime
Layer 3 → Dependencies
Layer 4 → App code

Docker reuses unchanged layers.

Example¶

docker pull node:18

If another image uses Node 18:

Docker reuses existing layers
Saves time & space

7️⃣ Dockerfile (How Images Are Built)¶

Concept¶

A Dockerfile:

Is a recipe
Describes how to build an image

Example Dockerfile¶

FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "server.js"]

Example Build Command¶

docker build -t my-app .

Meaning:

Docker reads instructions
Builds image layer by layer

8️⃣ Build Time vs Run Time¶

Concept¶

Phase	What Happens
Build	Dependencies installed
Run	App starts

Example¶

docker build -t my-app .
docker run my-app

You build once, run many times.

9️⃣ Container Isolation (Namespaces)¶

Concept¶

Each container has:

Its own process list
Its own network
Its own filesystem view

Example¶

docker exec -it container_id bash

Meaning:

You enter the container
It feels like a separate system
But kernel is shared

🔟 Networking (Ports Explained)¶

Concept¶

Containers have internal ports Host has external ports

Docker maps them.

Example¶

docker run -p 8080:80 nginx

Meaning:

Browser → Host:8080 → Container:80

1️⃣1️⃣ Storage (Why Containers Lose Data)¶

Concept¶

Containers are ephemeral:

Delete container → data lost

Solution: Volumes

Example¶

docker volume create mydata

docker run -v mydata:/data app

Meaning:

Data stored outside container
Safe even if container dies

1️⃣2️⃣ Environment Configuration¶

Concept¶

Same image, different environments:

Dev
Test
Prod

Config is injected at runtime.

Example¶

docker run -e DB_HOST=prod-db app

Meaning:

No code change
Only config changes

1️⃣3️⃣ Why Docker Starts Fast¶

Concept¶

Containers:

Don’t boot OS
Start like a process

Example¶

docker start container_id

Starts in milliseconds, not minutes.

1️⃣4️⃣ Docker for Multi-Service Apps¶

Concept¶

Modern apps = multiple services

Example:

Frontend
Backend
Database

Each runs in its own container.

Example Idea¶

docker run backend
docker run frontend
docker run database

Later → Docker Compose (automation)

1️⃣5️⃣ Docker in CI/CD¶

Concept¶

Docker enables:

Build once
Test once
Deploy everywhere

Example Flow¶

docker build
docker test
docker push
docker deploy

Same image moves across pipeline.

1️⃣6️⃣ Security (Simple View)¶

Concept¶

Docker provides:

Isolation
Resource limits

But:

Shared kernel
Needs best practices

Example¶

docker run --memory=512m app

Limits container RAM usage.

1️⃣7️⃣ Mental Model (MOST IMPORTANT)¶

Always think:

“Docker runs my app as an isolated process with its own filesystem and network, sharing the host kernel.”

If this clicks — Docker clicks.

✅ Final Summary¶

Image = Blueprint
Container = Running app
Dockerfile = Recipe
Engine = Worker
Registry = Storage
Kernel = Shared