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Building Custom Images ​

Sometimes you may want to build your own Docker image instead of using a pre-built one. For example, you might just need a lightweight environment with Python installed — or you may want to understand how images are built from scratch.

Let’s create a simple image that prints:

“Hello World from your Docker image”

1. Create a Project Folder ​

Create a new folder anywhere on your system, for example:

mkdir docker_hello
cd docker_hello

2. Create a Python Script ​

Inside this folder, create a file named hello.py and add the following line:

python
print("Hello World from your Docker image")

This is the small script that will run automatically when the container starts.

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3. Create a Dockerfile ​

In the same folder, create another file named Dockerfile (no extension) and add the following content:

dockerfile
# Start from the official Ubuntu base image
FROM ubuntu:20.04

# Install Python
RUN apt update && apt install -y python3

# Copy the Python script into the container
COPY hello.py /app/hello.py

# Set working directory
WORKDIR /app

# Run the script when the container starts
CMD ["python3", "hello.py"]

To create the file:

  1. Open Notepad (or any text editor).
  2. Paste the content above into the editor.
  3. Save it exactly as follows:
    • File → Save As…
    • File name: "Dockerfile" (with quotes, to ensure no extension is added).
    • Save as type: “All Files”.
    • Folder: Navigate to the folder where hello.py is located (e.g., docker_hello).
    • Click Save.

That’s it — the file will be saved without any extension.

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Here’s what this does:

  • FROM ubuntu:20.04 → starts from a clean Ubuntu system
  • RUN → installs Python
  • COPY → moves your hello.py script into the container
  • CMD → tells Docker what to execute when the container runs

4. Build the Image ​

Now build the image using:

bash
docker build -t hello_docker .

This command reads the Dockerfile and builds an image named hello_docker.

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5. Run the Container ​

Once the build is complete, run it using:

bash
docker run hello_docker

You should see the message:

Hello World from your Docker image

That’s it — you’ve successfully built and run your first custom Docker image!

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Summary ​

  • Dockerfile defines what goes inside your image
  • docker build creates the image
  • docker run executes it

This small exercise shows how containers can package even simple scripts in a reproducible way — the same steps apply when scaling up to full simulation environments later.

Next, we will explore docker volumes and how to manage data between your host system and containers.