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Rahul Joshi
Rahul Joshi

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Day 6 — Docker & Containerization

#masterclassdevsecops #webdev #docker #container

Modern software development has changed completely.

Applications are no longer deployed directly on servers like before.

Today, companies package applications into containers so they can run consistently anywhere:

  • Developer laptop
  • Cloud servers
  • Kubernetes clusters
  • CI/CD pipelines
  • Edge infrastructure

🔗 Resources

🧠 What is a Container?

A container is a lightweight isolated environment that includes:

  • Application code
  • Runtime
  • Libraries
  • Dependencies
  • Environment variables
  • System tools

Everything required to run the application is packaged together.

That means:

  • Same behavior everywhere
  • Faster deployments
  • Portable infrastructure
  • Easy scaling

⚔️ Containers vs Virtual Machines

This is one of the most important concepts in DevOps.

Both Containers and VMs isolate applications — but in very different ways.

🖥️ Virtual Machines (VMs)

A VM includes:

  • Full Operating System
  • Guest Kernel
  • Hypervisor
  • Application

Problems with VMs

  • High memory usage
  • Slow startup time
  • Large storage consumption
  • Less efficient scaling

🐳 Containers

Container

Containers share the host OS kernel.

They only package:

  • Application
  • Dependencies
  • Runtime

This makes them:

  • Lightweight
  • Fast
  • Portable
  • Efficient

📊 Containers vs VMs Comparison

Feature Containers Virtual Machines
Startup Time Seconds Minutes
Size MBs GBs
Performance Near-native Heavy overhead
Isolation Process-level Full OS-level
Resource Usage Low High
Portability Excellent Moderate

📦 What is Docker?

Docker is a platform used to:

  • Build applications
  • Package applications
  • Ship applications
  • Run applications in isolated environments called containers

Docker ensures:

“It works the same everywhere.”

This solves the classic developer problem:

“It works on my machine.”

🏗️ Docker Architecture

Docker mainly consists of 3 components:

Docker Registry

1️⃣ Docker Client

The CLI where commands are executed.

Example:

docker build
docker run
docker ps
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2️⃣ Docker Daemon

The background service (dockerd) responsible for managing:

  • Containers
  • Images
  • Networks
  • Volumes

3️⃣ Docker Registry

A place where Docker images are stored.

Popular registries include:

  • Docker Hub
  • GitHub Container Registry
  • AWS ECR
  • Google Artifact Registry

🧱 What is a Docker Image?

A Docker Image is a:

Read-only blueprint used to create containers.

It contains:

  • Application code
  • Dependencies
  • Runtime
  • Configuration
  • Libraries

🧠 Simple Analogy

Concept Real World Example
Docker Image Recipe
Docker Container Cooked Food

The image is the template.

The container is the running instance.

🔍 Popular Docker Images

Some commonly used images:

nginx
ubuntu
node
python
mysql
redis
postgres
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Pull an image:

docker pull nginx
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Run a container:

docker run nginx
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📝 What is a Dockerfile?

A Dockerfile is a text file containing instructions used to build a Docker image.

This is where containerization begins.

🧱 Example Dockerfile 

FROM node:20-alpine

WORKDIR /app

COPY package.json package-lock.json ./

RUN npm ci --only=production

COPY . .

EXPOSE 3000

CMD ["node", "app.js"]
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🔬 Dockerfile Explained

FROM

Defines the base image.

FROM node:20-alpine
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WORKDIR

Sets the working directory inside the container.

WORKDIR /app
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COPY

Copies files from local system into the container.

COPY . .
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RUN

Executes commands during image build.

RUN npm install
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EXPOSE

Documents which port the application uses.

EXPOSE 3000
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CMD

Defines the default startup command.

CMD ["node", "app.js"]
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🚀 Building Docker Images

Build image:

docker build -t myapp .
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Explanation

Part Meaning
docker build Build Docker image
-t Tag image
myapp Image name
. Current directory

▶️ Running Containers

Run container:

docker run -p 3000:3000 myapp
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🔍 Understanding Port Mapping 

-p 3000:3000
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Means:

Host Machine Container
3000 3000

Access app on:

http://localhost:3000
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📂 Containerizing a Node.js Application

📁 Project Structure 

project/
│
├── app.js
├── package.json
└── Dockerfile
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📄 app.js 

const express = require("express");
const app = express();

app.get("/", (req, res) => {
  res.send("Docker is working!");
});

app.listen(3000);
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📄 package.json 

{
  "dependencies": {
    "express": "^4.18.2"
  }
}
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📄 Dockerfile 

FROM node:20-alpine

WORKDIR /app

COPY package*.json ./

RUN npm ci

COPY . .

EXPOSE 3000

CMD ["node", "app.js"]
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🏗️ Build the Image 

docker build -t node-docker-demo .
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▶️ Run the Container 

docker run -p 3000:3000 node-docker-demo
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Now open:

http://localhost:3000
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You should see:

Docker is working!
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📦 Docker Image Layers

Docker images are layered.

Each instruction creates a new layer.

Example:

FROM ubuntu:24.04
RUN apt update
RUN apt install nginx
COPY . .
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Each line becomes a cached layer.

This makes builds:

  • Faster
  • Efficient
  • Reusable

⚡ Docker Caching

Docker rebuilds only changed layers.

That’s why Dockerfiles should be optimized carefully.

✅ Cache-Friendly Pattern 

COPY package.json .
RUN npm install

COPY . .
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Why?

Dependencies are cached separately.

If only source code changes, Docker skips reinstalling packages.

❌ Bad Pattern 

COPY . .
RUN npm install
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Any file change invalidates cache.

Build becomes slower.

📁 Docker Volumes

Containers are ephemeral.

Data disappears after container removal.

Volumes solve this problem.

Create Volume 

docker volume create mydata
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Use volume:

docker run -v mydata:/data nginx
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🌐 Docker Networking

Containers communicate using Docker networks.

Create network:

docker network create mynetwork
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Run containers inside same network.

Useful for:

  • APIs
  • Databases
  • Microservices
  • Internal communication

🧹 Essential Docker Commands

📦 Images 

docker images
docker pull nginx
docker build -t myapp .
docker push username/myapp:v1
docker rmi image_id
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🐳 Containers 

docker ps
docker ps -a
docker stop container_id
docker rm container_id
docker logs -f container_id
docker exec -it container_id sh
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🧽 Cleanup 

docker system prune
docker volume prune
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🔥 Why Docker Became So Popular

Docker transformed software deployment because it provides:

  • Environment consistency
  • Faster deployments
  • Infrastructure portability
  • Easy CI/CD integration
  • Better scalability
  • Microservices support
  • Cloud-native compatibility

☁️ Docker in Modern DevOps

Docker is now everywhere.

Technology Docker Usage
Kubernetes Runs containers
CI/CD Build & deploy
Cloud Platforms Container hosting
DevSecOps Isolated workloads
Microservices Service packaging

🔐 Docker Security Basics

Important security practices:

  • Use minimal base images
  • Avoid running as root
  • Scan images regularly
  • Keep dependencies updated
  • Use signed images
  • Remove unused containers/images

🏭 Real-World Docker & K8s Workflow

Docker WorkFlow

🚀 Docker Best Practices

✅ Use Smaller Images

Prefer:

FROM node:20-alpine
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Instead of huge base images.

Smaller images:

  • Pull faster
  • Reduce attack surface
  • Save storage

✅ Always Use .dockerignore

Example:

node_modules
.git
.env
*.log
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This reduces build size and prevents accidental secret leaks.

✅ Multi-Stage Builds

Use one stage for building.

Use another minimal stage for production.

✅ Tag Images Properly

Bad:

latest
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Good:

v1.0.2
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✅ Don’t Run Containers as Root

Example:

RUN adduser -S appuser
USER appuser
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🎭 Multi-Stage Build Example 

# Stage 1 — Build
FROM node:20-alpine AS builder

WORKDIR /app

COPY package*.json ./

RUN npm ci

COPY . .

RUN npm run build

# Stage 2 — Production
FROM node:20-alpine

WORKDIR /app

COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules

EXPOSE 3000

CMD ["node", "dist/index.js"]
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📚 Final Thoughts

Docker completely changed how modern applications are built and deployed.

Understanding:

  • Containers
  • Docker Images
  • Dockerfiles
  • Layers
  • Volumes
  • Networking

is now a fundamental engineering skill.
Because modern infrastructure runs on containers.

most popular alternatives to Docker for containerization and container runtime workflows:

  • Podman
  • containerd
  • CRI-O

🎯 Quick Recap

Concept Meaning
Container Isolated runtime
Docker Image Blueprint/template
Dockerfile Build instructions
Container Running image instance
Volume Persistent storage
Network Container communication

Top comments (2)

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f0rty profile image
Xavi •

Love how detailed it is 🤗, thank you for sharing!

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17j profile image
Rahul Joshi •

Thank you so much for the kind words! 🤗 Glad you found the detailed breakdown helpful.

Just a heads-up, this is part of my ongoing 30 Days DevSecOps Journey series. I’m deep-diving into security automation, CI/CD, and cloud-native practices every day.

If you want to follow along with the upcoming days, feel free to follow me here on Dev.to! Also, you can check out the complete roadmap, source code, and configurations in my GitHub repository. Do star ⭐ and fork it to experiment with the setups yourself!

Looking forward to your thoughts on the next posts! 🚀