🐳 How to Create a DockerFile to Build an Image – A Step-by-Step Guide with FAQs

Creating a Docker image starts with writing a DockerFile, which is a text document containing all the commands needed to assemble the image. Once you have a Dockerfile ready, you can use it to build your own custom Docker image tailored to your application’s needs.

In this article, we’ll explore how to create a Dockerfile from scratch, walk through each important instruction, and answer frequently asked questions.

---

📦 What is a DockerFile?

A Dockerfile is a script composed of various commands and instructions to define the contents and behavior of a Docker image. When you run the docker build command, Docker reads the DockerFile and generates an image based on it.

---

🛠️ Basic Structure of a DockerFile

A DockerFile consists of multiple instructions, each performing a specific role. Here’s the typical structure:

# Base image
FROM ubuntu:20.04

# Maintainer information (optional)
LABEL maintainer="yourname@example.com"

# Install dependencies
RUN apt-get update && apt-get install -y \
    python3 \
    python3-pip

# Set working directory
WORKDIR /app

# Copy files from host to container
COPY. /app

# Install Python packages
RUN pip3 install -r requirements.txt

# Expose a port
EXPOSE 5000

# Default command to run
CMD ["python3", "app.py"]

---

🔍 Explanation of Each DockerFile Instruction

Instruction	Description
FROM	Specifies the base image to use. It’s the foundation of your image.
LABEL	Optional metadata such as maintainer name, version, etc.
RUN	Executes commands inside the image at build time (e.g., installing software).
WORKDIR	Sets the working directory inside the container.
COPY	Copies files or folders from your local system to the image.
EXPOSE	Indicates the port the container listens on at runtime.
CMD	Provides the default command to run when the container starts.

---

🧪 Example: Create a Simple Flask App Image

Let’s say you have a small Flask application with the following structure:

/flask-app
│
├── app.py
├── requirements.txt
└── DockerFile

📄 app.py

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "Hello from Docker!"

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

📄 requirements.txt

Flask==2.1.2

📄 DockerFile

To create a file named DockerFile, run:

vi DockerFile

This opens the vi editor.

FROM python:3.9-slim

WORKDIR /app

COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

EXPOSE 5000

CMD ["python", "app.py"]

✅ To build and run this Docker image:

docker build -t flask-app .
docker run -p 5000:5000 flask-app

Now, open your browser and go to http://localhost:5000 to see your Flask app in action!

---

💡 Best Practices for Writing DockerFiles

• 🧼 Minimize layers: Combine multiple RUN commands using && to reduce image layers.
• 🚀 Use slim base images: Choose minimal base images like python:3.9-slim or alpine for performance.
• 🔐 Avoid hardcoding secrets: Never store passwords or API keys in DockerFiles.
• 📁 Use .dockerignore: Exclude unnecessary files from your image using a .dockerignore file.

---

🎯 Final Thought

Writing a DockerFile is the first and most essential step in creating a Docker image. With a well-structured DockerFile, you can easily automate the setup of your applications and deploy them consistently across environments.

Once you understand each instruction and follow best practices, you’ll be able to build lean, efficient, and powerful Docker images.

---

❓ Frequently Asked Questions (FAQ)

1. 🔄 What’s the difference between CMD and ENTRYPOINT?

• CMD sets default arguments for the container.
• ENTRYPOINT defines the main executable. You can combine both for flexible control.

---

2. 🔐 Can I pass environment variables in a DockerFile?

Yes! You can use the ENV instruction:

ENV API_KEY=12345

You can also override it at runtime:

docker run -e API_KEY=67890 my-image

---

3. 🧹 How do I reduce Docker image size?

• Use a smaller base image (alpine, slim).
• Remove unnecessary packages.
• Use --no-cache and clean up files after installation.

---

4. 🔁 How do I rebuild an image after changes?

Use the --no-cache option to force a fresh build:

docker build --no-cache -t my-image .

---

5. 🛑 Why isn’t my CMD or ENTRYPOINT working?

Make sure:

• You’re using JSON array syntax: ["python", "app.py"]
• There’s no syntax conflict between CMD and ENTRYPOINT.

---