Dockerfile, Custom Images, and Environment Variables Explained for Beginners

Dockerfile, Custom Images, and Environment Variables Explained for Beginners

Docker is one of the most important technologies in modern DevOps, cloud computing, and software deployment. After learning Docker Images and Containers, the next step is understanding Dockerfiles, building custom images, and using environment variables. 
Learn Dockerfile, custom Docker image creation, and environment variables with practical examples. A beginner-friendly Docker guide for DevOps and System Engineers.

In this guide, you'll learn these concepts with practical examples.

What is a Dockerfile?

A Dockerfile is a text file that contains instructions for building a Docker image. Instead of manually configuring containers every time, you can define all steps inside a Dockerfile and create reproducible images.

Example:

dockerfile
FROM nginx
COPY index.html /usr/share/nginx/html/index.html
  

• * FROM nginx uses the Nginx image as the base image.
• * COPY copies a webpage into the Nginx web server directory.

Docker reads these instructions and builds a custom image.

 Common Dockerfile Instructions

FROM

• Specifies the base image.
• FROM ubuntu

 WORKDIR

Sets the working directory inside the container.

• WORKDIR /app

 COPY

Copies files from the host machine into the container.

• COPY . .

RUN

Executes commands during image creation.

• RUN npm install

CMD

Defines the default command executed when the container starts.

• CMD ["node","server.js"]

Building Custom Docker Images

Let's create a simple custom image.

Project Structure

text
project/
│
├── Dockerfile
├── index.html

Dockerfile

• FROM nginx
• COPY index.html /usr/share/nginx/html/index.html

Build the Image

Run the following command inside the project directory:

• docker build -t mywebsite . 

 
Explanation:
 

• docker build creates a Docker image.
• -t assigns a tag or image name.
• mywebsite is the image name.
• .  represents the current directory.

Verify the Image

• docker images

You should see:

• REPOSITORY
• mywebsite

Run the Custom Image

• docker run -d -p 8080:80 mywebsite

Open:

• http://localhost:8080

Your custom webpage should now be visible in the browser.

What Are Environment Variables?

Environment variables allow you to pass configuration values to containers without modifying application code.

This makes applications easier to manage and deploy.

Example:

• docker run -e APP_NAME=HackingTruth nginx

Here:

 

• APP_NAME=HackingTruth

is an environment variable available inside the container.

Multiple Environment Variables

docker run \
-e DB_HOST=localhost \
-e DB_USER=root \
-e DB_PASSWORD=password \
myapp

 

This approach is commonly used for:

• * Database credentials
• * API keys
• * Application settings
• * Deployment configuration

Defining Environment Variables in Dockerfile


You can also define variables directly inside a Dockerfile.

• FROM ubuntu
• ENV APP_NAME=HackingTruth
• ENV VERSION=1.0

Build the image:

• docker build -t myapp .

Run the container:

• docker run myapp

The environment variables will be available inside the container.

Why Use Dockerfiles and Environment Variables?

Dockerfiles provide:

• * Consistent deployments
• * Reproducible environments
• * Easier automation

Environment variables provide:

• * Better configuration management
• * Improved security practices
• * Easier deployment across environments

Together, they form the foundation of modern containerized applications.

Conclusion

Understanding Dockerfiles, custom image creation, and environment variables is essential for anyone learning Docker, DevOps, Cloud Computing, or System Administration.

By mastering these concepts, you'll be able to create reusable Docker images, automate deployments, and configure applications efficiently. These skills are widely used in real-world production environments and are frequently asked about in DevOps and System Engineer interviews.