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Getting Started with Docker: Python “Hello World” Example

Get a quick introduction to containerisation with Docker from one of our Research Software Engineers (RSE). Docker packages apps and their dependencies and are an ideal way to solve the "works on my machine" problem. Learn about Docker's main perks: consistency, portability, isolation, and scalability and there are links to extra resources if you want to learn more.

What is Containerisation?

Software development often faces the challenge: "it works on my machine, but not yours." This usually happens because of differences in operating systems, libraries, or other dependencies. Containerisation solves this by packaging an application along with its entire runtime environment – all necessary code, libraries, and configuration files – into a single, standardised unit called a container. This ensures the application runs consistently and reliably, regardless of where the container is deployed.

What is Docker?

Docker is currently the most popular platform for creating, deploying, and managing containers. It provides tools to build container images (blueprints for containers) and run them efficiently. Compared to traditional virtual machines, Docker containers are more lightweight, start faster, and use fewer system resources because they share the host operating system's kernel.

Access and Usage

Docker is available from its official website, docker.com, where you’ll find both free and paid subscription options. For most individual developers and small-scale projects, the free Docker Desktop tier is more than enough. Getting started is straightforward: download Docker Desktop from the site and install it on Windows, macOS, or Linux.

Why Use Docker?

  • Consistency: Ensures applications run identically across development, testing, and production environments.
  • Portability: Containers can be easily moved and run on any machine with Docker installed.
  • Isolation: Applications inside containers run independently, preventing conflicts.
  • Scalability: Easily scale applications by running multiple instances of a container.

Docker "Hello World" Example with Python

Let's create a simple Docker container for a Python "Hello World" application.

Step 1: Create the Python Script

Create a file named app.py:

# app.py 

print("Hello, Docker World!")

Step 2: Create the Dockerfile

A Dockerfile is a text file containing instructions to build a Docker image. Create a file named Dockerfile (this is the standard convention):

# Dockerfile 

# Use an official lightweight Python image 

FROM python:3.12-slim 

 

# Set the working directory inside the container 

WORKDIR /app 

 

# Copy the Python script into the container's working directory 

COPY app.py . 

 

# Specify the command to run when the container starts 

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

Step 3: Build the Docker Image

In your terminal, navigate to the directory containing app.py and Dockerfile, then run:

# Build the image and tag it as 'hello-docker' 

docker build -t hello-docker .

Step 4: Run the Docker Container

Execute the following command to run your newly built image as a container:

docker run hello-docker

You should see the output:

Hello, Docker World!

Beyond "Hello World": Scaling Up

While this example is basic, Docker's power shines in more complex scenarios. For applications with multiple components, docker-compose allows you to define and manage multi-container applications easily. This simplifies handling different sets of dependencies for various projects running on the same system, making development and deployment significantly more manageable.

Learn More

If you would like to learn more about applying Docker to your project, Research IT is hosting the Introduction to Docker course on 6-7 October 2025. The course notes, detailing the content, are available already. Have a look at the range of courses offered by Research IT and sign up today.