🛳️ Docker Series: Episode 4 — Dockerfile for Beginners: Build Your Own Container Image

August 26, 2025

This content originally appeared on DEV Community and was authored by Yash Sonawane

“Ready to become a Docker chef? In this episode, we’ll write your first Dockerfile — the recipe that creates your own custom container. Simple, step-by-step, with a real app. Let’s cook! “

What is a Dockerfile?

A Dockerfile is like a recipe card for Docker. It tells Docker exactly how to build an image from scratch.

Just like:

A cake recipe includes ingredients and steps
A Dockerfile includes base image, dependencies, code, and commands

Once Docker reads the Dockerfile, it builds a Docker image, which you can then run as a container.

Let’s Build Your First Dockerfile

We’ll dockerize a basic Node.js app.

Project Structure:

my-node-app/
├── Dockerfile
├── package.json
└── index.js

`index.js` (your app):

const http = require('http');

const server = http.createServer((req, res) => {
  res.end('Hello from Docker!');
});

server.listen(3000, () => console.log('Server running on port 3000'));

`package.json`:

{
  "name": "my-node-app",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "start": "node index.js"
  }
}

Writing the Dockerfile

Now create a file called Dockerfile (no extension!) and add this:

# Base image
FROM node:18

# Working directory
WORKDIR /app

# Copy files
COPY package*.json ./

# Install dependencies
RUN npm install

# Copy app source
COPY . .

# Expose port
EXPOSE 3000

# Start the app
CMD ["npm", "start"]

Build & Run the Docker Image

Build the image:

docker build -t my-node-app .

Run the container:

docker run -d -p 3000:3000 my-node-app

Open your browser:

Visit http://localhost:3000 — you’ll see: Hello from Docker!

What Did We Just Do?

FROM: Sets the base environment (Node.js)
WORKDIR: The folder where your code lives inside the container
COPY: Moves your code into the container
RUN: Installs dependencies
EXPOSE: Lets Docker know what port the app uses
CMD: Tells Docker how to start the app

Bonus Tips

Use .dockerignore to skip node_modules, logs, etc.
Always pin your base image (node:18 not just node)
Keep Dockerfiles short, clean, and readable

Up Next: Docker CLI Mastery

In Episode 5, we’ll explore:

Most used Docker commands
How to manage images and containers
Tricks to make your Docker life easier

Let’s Build Together

Have you written your first Dockerfile? Did it work? Run into any errors?

Comment below — I’m here to help every beginner succeed

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Next: “Docker CLI Cheat Sheet — 15 Commands You’ll Actually Use”