Project 1.1: Hello Docker

Everyone talks about Docker like it’s some rite of passage. You hear “just containerize it” thrown around in job descriptions, tutorials, Stack Overflow answers. So I finally sat down and did the most basic thing possible: made it print “Hello from Docker!” and run cleanly.

Glamorous? No. Necessary? Absolutely.

The Goal

Get one container running. A Dockerfile, a Python script, a docker run that works. No overthinking — just enough to understand what’s actually happening when people talk about images and containers.

What I Built

A minimal Docker setup that:

• Builds an image from a Dockerfile
• Runs a Python script inside the container
• Outputs something to the terminal (and exits without drama)

Tech Stack

• Docker — Container runtime
• Python — Simple script execution

Implementation

Dockerfile

FROM python:3.11-slim
 
WORKDIR /app
 
COPY hello.py .
 
CMD ["python", "hello.py"]

Python Script

# hello.py
print("Hello from Docker!")
print("Container is working.")

Commands

# Build the image
docker build -t hello-docker .
 
# Run the container
docker run hello-docker

What I Learned

• Images vs containers: An image is the blueprint; a container is the running instance. docker build creates the image, docker run spins up a container from it. Obvious in hindsight, confusing when you start.
• Dockerfile syntax: FROM picks your base, WORKDIR sets your directory, COPY moves files in, CMD defines what runs. That’s the whole thing for a basic script.
• Layer caching: Order matters. Put things that change often (your script) at the bottom. Docker caches layers from top to bottom and rebuilds from the first change onward.
• Image tagging: The -t hello-docker flag is just a name so you don’t have to remember a hash. Use it.

Challenges

Challenge: Docker Desktop was eating an embarrassing amount of RAM on my Mac. Solution: Adjusted resource limits in Docker Desktop settings. 4GB is plenty for this kind of work. I had left the defaults which were way too generous.

Challenge: Edited the Python script, ran the container, nothing changed. Solution: docker build doesn’t watch for file changes. You have to rebuild manually every time. Later I’ll use docker-compose to make this less painful, but for now — muscle memory.

Result

$ docker run hello-docker
Hello from Docker!
Container is working.

It ran. It printed. It exited cleanly. That’s it — that’s the whole win for project 1.1. And honestly? That clean exit felt satisfying after fumbling through the concepts. Foundation laid.

Related

• de-project-1-2-postgresql-in-docker
• Data-Engineering-Fundamentals
• Python-for-Data-Engineering
• fixing-immich-installation

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Project: 1.1 of 28 | Month: January | Hours: ~3h GitHub: 1-1_Hello-Docker