Run Your First Docker Container: Understanding the Magic Behind the Scenes

Running your very first Docker container is a magical moment.

Most Docker tutorials start with these two simple commands:

docker pull hello-world
docker run hello-world

And Docker responds:

That’s it. Success! 🎉

It works so fast and so smoothly that you might miss what’s really happening in the background. It prints Hello from Docker! and you’re left thinking: “Wait... that’s it?”

This guide isn't just about running a command. We're going behind the scenes to understand exactly how Docker works the first time you run a container. We’ll use a simple diagram and explain each step clearly, so Docker feels less like magic and more like a powerful tool in your development journey.

Let’s break it down.

🧠 What Happens When You Run a Docker Command?

While the commands seem simple, a lot is happening under the hood.

👤 1: You (the User)

You are the user sitting at your machine. You’ve installed Docker and are ready to run containers maybe MySQL, PostgreSQL or a simple hello-world image.

⚙️ 2: Docker Engine

What you have actually installed is the Docker Engine — the underlying tech that handles the build, run, and lifecycle of containers.

Think of it as the core engine that interprets your commands and talks to other Docker components.

🗂 3: Local Docker Repository (aka Cache)

When you run docker run hello-world, Docker first checks if this image (hello-world) is already present in your local image cache.

If it’s already available locally, Docker skips downloading and moves straight to running it.

🌐 4: Docker Hub (Cloud Repository)

If the image is not found locally, Docker reaches out to Docker Hub — the official public registry where developers share and download Docker images.

It downloads the image (only once) and stores it locally. From next time, it won’t re-download unless there's a version mismatch or cache clear.

docker pull vs docker run

Let’s demystify the difference:

✅ docker pull hello-world

• Downloads the hello-world image from Docker Hub.

• Stores it locally.

• Doesn’t run it.

  

✅ docker run hello-world

• First checks if the image exists locally.

• If not, it pulls the image (like docker pull).

• Then creates a container from that image and runs it.

  

So, in a way:

docker run hello-world

is the same as:

docker pull hello-world
docker create hello-world
docker start <container_id>

💡 Did you know:
The hello-world image has been downloaded over 1 billion times on Docker Hub! It’s that iconic.

Understanding Images vs Containers

• Image: A read-only template with the application code and environment.

• Container: A running instance of an image (like a lightweight virtual machine).

You can run multiple containers from the same image.

🖥 Visualizing the Flow

Let’s understand the flow:

1. 🧑‍💻 You run docker run hello-world.

2. 🔍 Docker Engine checks the local cache (LocalHub).

3. 📦 If not found, it pulls the image from Docker Hub.

4. 🧱 It creates a container from the image.

5. ▶️ The container runs and prints: Hello from Docker!

6. 🛑 The container exits after execution (since the program is just a print statement).

You can view all this using Docker Desktop (GUI) or docker ps -a to list all containers (even exited ones).

🎯 Bonus: Clean-Up and Retesting

Let’s say you want to clean everything and try again:

🛑 Stop and Remove Containers:

docker ps -a           # List all containers
docker stop <id>       # Stop running container
docker rm <id>         # Remove stopped container

🧹 Remove the Image:

docker rmi hello-world

Now when you run docker run hello-world, it will trigger a fresh download.

🎉 You Did It!

If you’ve followed along, you’ve not only run your first Docker container, but you've also understood exactly how and why it worked.

You now know:

• What Docker Engine is.

• How images and containers relate.

• The role of Docker Hub.

• What pull vs run really means.

I hope you found this blog helpful and informative. I would love to hear your thoughts and feedback. If you have any questions or suggestions, feel free to reach out.

Thanks for reading! 🙌

Happy learning! 🐳