Dockerfile Explained: How to Build Custom Docker Images
Dockerfiles are the foundation for creating your own Docker images. Instead of downloading pre-made containers, you can define exactly what you need — from system packages to application code — all in a single file. Let’s learn how to build custom docker images step by step.
What is a Dockerfile?
A Dockerfile is a plain text file that contains instructions to build a Docker image. Each line in the file is a command that defines what your environment should look like.
It’s like writing a recipe for your app environment — once defined, you can rebuild it anytime, anywhere.
Steps To Build Custom Docker Images
Simple Example: Building a Python App
Let’s say you have a basic Python app (app.py) with this content:
# app.py
print("Hello from Docker!")
Now, here’s how to write a Dockerfile to run this app:
# Use the official Python base image
FROM python:3.10-slim
# Set the working directory inside the container
WORKDIR /app
# Copy the app.py file to the container
COPY app.py .
# Define the default command to run
CMD ["python", "app.py"]
Make sure the folder structure is as follows:
your-folder/
├── Dockerfile
└── app.py
Step-by-Step Explanation
| Line | Purpose |
FROM | Sets the base image (in this case, Python 3.10) |
WORKDIR | Sets /app as the working directory inside the container |
COPY | Copies the app.py file into the container |
CMD | Tells Docker what to run when the container starts |
How to Build the Image
In the same directory as your Dockerfile and app.py, run:
docker build -t my-python-app .
This command builds a Docker image called my-python-app.
Run the Container
Now let’s run it:
docker run my-python-app
Expected output:
Hello from Docker!
Tips for Custom Dockerfiles
- Use
.dockerignoreto exclude files/folders from your image (like.git,__pycache__, etc.) - Use
RUNfor installing dependencies, e.g.:
RUN pip install requests
- Combine layers for efficiency:
RUN apt-get update && apt-get install -y curl
Clean Up (Optional)
Remove the container after testing:
docker container prune
Remove the image if no longer needed:
docker rmi my-python-app
Dockerfiles give you full control over your application environment, making it easy to share, deploy, and scale. With just a few lines, you can build custom containers tailored to your project’s needs.
You can read more useful articles like How To Install Docker On Ubuntu.
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