Building and Updating Docker Containers: A Practical Hands-On Guide for Beginners

Source: Dev.to

Introduction

In today’s fast‑paced development environment, consistency and portability are critical. Applications must run reliably across different machines, environments, or even in the cloud. Docker provides a solution by packaging your application along with its dependencies into a container, ensuring it behaves the same everywhere.

In this hands‑on guide, you’ll learn how to containerize a simple Node.js “Todo List” application. You don’t need prior experience with Node.js — the focus is on understanding how Docker builds, packages, and runs applications.

By the end of this tutorial, you’ll be able to:

• Build a Docker image using a Dockerfile
• Run your application inside a container
• Expose your app to your local host
• Understand how images, containers, and layers interact

Prerequisites

Ensure you have the following tools installed:

• Docker Desktop (latest version)
• Git (for cloning the sample app)
• VS Code or another code editor

These tools allow you to build and run containers smoothly.

Step 1: Prepare the Application

Open a new folder in VS Code and name it container.

Open a terminal in that folder.

Clone the sample Node.js application:

git clone https://github.com/docker/getting-started-app.git

You’ll see the following folder structure:

getting-started-app/
│── .dockerignore
│── package.json
│── README.md
│── spec/
│── src/
└── yarn.lock

This is the source code you will containerize.

Step 2: Create the Dockerfile

In the getting-started-app folder, create a file named Dockerfile and add:

# syntax=docker/dockerfile:1

FROM node:lts-alpine
WORKDIR /app
COPY . .
RUN yarn install --production
CMD ["node", "src/index.js"]
EXPOSE 3000

Explanation

• FROM – selects a lightweight Node.js base image
• WORKDIR – sets the working directory inside the container
• COPY – transfers your source code into the container
• RUN – installs production dependencies
• CMD – defines the command to start the app
• EXPOSE – documents the port the app listens on

Step 3: Build the Docker Image

Navigate to the project folder:

cd /path/to/getting-started-app

Build the image:

docker build -t getting-started .

What happens:

• Docker downloads the Node.js base image (if not already present)
• Copies your app files into the image
• Installs dependencies
• Packages everything into a Docker image named getting-started

Step 4: Run Your Container

Start the container:

docker run -d -p 127.0.0.1:3000:3000 getting-started

Flags explained

• -d – runs the container in the background (detached)
• -p HOST:CONTAINER – maps container port 3000 to your local port 3000

Visit http://127.0.0.1:3000 in your browser. Your Todo List app should be running. Add tasks, mark them complete, and see how everything works inside the container.

Step 5: Verify Running Containers

Check which containers are active:

docker ps

You’ll see the container ID, image name, port mappings, and status — confirming that your app is active.

Step 6: Update the Application

Suppose you want to change the “empty text” message to:

You have no todo items yet! Add one above!

Edit src/static/js/app.js and update line 56:

- No items yet! Add one above!
+ You have no todo items yet! Add one above!

Rebuild the Docker image with the updated code:

docker build -t getting-started .

Start a new container using the updated image:

docker run -d -p 127.0.0.1:3000:3000 getting-started

Your changes are now reflected in the running application.