Pull, Build an image using a Dockerfile and Push the image to Docker Hub.

Pull an image from Docker Hub and run it as a container

🧰 Prerequisites

• You have Docker installed on your machine.
  • Check with: docker --version
  • If not installed, follow the official Docker installation guide for your OS: https://docs.docker.com/get-docker/

  sudo apt install docker.io -y

✅ Add user to the docker group for avoid typing sudo on every command.

Run the following command once with sudo:

sudo usermod -aG docker $USER

Then, log out and log back in (or reboot) for the group changes to take effect.

For immediately:

newgrp docker

🔹 Pull an Image from Docker Hub

Use the docker pull command:

docker pull hello-world

💡 Explanation:

• docker: The main Docker CLI command.
• pull: Tells Docker to download an image from Docker Hub.
• hello-world: The name of the image to pull.

This pulls the official hello-world image, which is small and used for testing Docker installations.

🔹 List Images on Your Machine

After pulling the image, check if it was successfully downloaded:

docker images

💡 Output will look like this:

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE
hello-world   latest    d1165f22117a   2 months ago   13.3kB

💡 Explanation:

• REPOSITORY: Name of the image (hello-world)
• TAG: Version tag (default is latest)
• IMAGE ID: Unique identifier for the image
• SIZE: Size of the image

🔹 Run the Image as a Container

Now, run the image using the docker run command:

docker run hello-world

💡 Explanation:

• run: Tells Docker to create and start a new container from the specified image.
• hello-world: The image to use for creating the container.

💡 What Happens?

• Docker checks if the image exists locally.
• If yes (which it does), it starts a new container from that image.
• The container runs the default command defined in the image (in this case, prints a welcome message).

You should see output like:

Hello from Docker!
This message shows that your installation appears to be working correctly.
...

🔹 List Running Containers

To see containers currently running:

docker ps

But since hello-world exits immediately after printing the message, it won’t show up here.

To see all containers, including stopped ones:

docker ps -a

💡 Output will look like:

CONTAINER ID   IMAGE         COMMAND    ...   STATUS                     NAMES
abc123xyz      hello-world   "/hello"   ...   Exited (0) 10 seconds ago   youthful_banach

🔹 Optional: Remove the Container

If you want to clean up:

docker rm abc123xyz

Replace abc123xyz with your actual container ID or name.

🔹 Optional: Remove the Image

To remove the image from your system:

docker rmi hello-world

📌 Summary of Commands

🎯 Example with Another Image (nginx Web Server)

Let’s try something more practical — running a real web server:

docker run -d -p 8080:80 nginx

💡 Explanation:

• -d: Run container in detached mode (in background)
• -p 8080:80: Map port 8080 (your computer) to port 80 (inside the container)
• nginx: Official NGINX image

Now open your browser and go to:
👉 http://localhost:8080
You should see the NGINX welcome page!

If you’re encountering this port error:

Bind for 0.0.0.0:8080 failed: port is already allocated

🔍 What does this mean?

This error occurs because port 8080 on your machine is already being used by another process, so Docker can’t bind to it.

✅ Step-by-Step Fix

🪛 Find the Process Using Port 8080

Run this command in your terminal:

sudo lsof -i :8080

Or if you don’t have lsof, use:

sudo netstat -tulpn | grep :8080

Example Output:

COMMAND   PID   USER    FD   TYPE DEVICE SIZE/OFF NODE NAME
node    12345   user   20u  IPv6 123456      0t0  TCP *:8080 (LISTEN)

Here, the process with PID 12345 is using port 8080.

🚫 Stop the Conflicting Process

Use the kill command with the PID from above:

sudo kill -9 12345

Replace 12345 with the actual PID you found.

⚠️ Be careful not to kill important system processes. Make sure you know what the process is before killing it.

▶️ Try Running the Docker Command Again

Now try starting NGINX again:

docker run -d -p 8080:80 nginx

It should now work and return a container ID like:

a14767df4f989392399558bee003840abe43350569edbe2c121ca304a683f1af

You can verify it’s running with:

docker ps

And visit http://localhost:8080 in your browser.

🔄 Alternative: Use a Different Host Port

If you don’t want to stop the existing service using port 8080, you can map NGINX to a different port on your host — for example, 8000:

docker run -d -p 8000:80 nginx

Then open:

👉 http://localhost:8000

🧹 Optional: Clean Up Unused Containers/Ports

To avoid future conflicts, clean up unused containers:

docker stop $(docker ps -aq)
docker rm $(docker ps -aq)

⚠️ This will remove all running/stopped containers! Only do this if you’re okay with that.

Build an image using a Dockerfile and Run the image as a container

📁 Create a Project Directory

Open your terminal and create a new folder for your project:

mkdir my-docker-app
cd my-docker-app

This will be the working directory where you’ll create your Dockerfile and application files.

📄 Create a Simple Application

For this example, we’ll create a very simple Python web app using Flask.

🔹 Create a file named app.py:

nano app.py

Now paste the following code:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello from Docker!"

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=5000)

This is a basic Flask app that responds with "Hello from Docker!" when accessed via a browser.

🐳 Create a Dockerfile

A Dockerfile is a script containing instructions to build a Docker image.

Create the Dockerfile:

nano Dockerfile

Now paste the following code:

# Use an official Python runtime as the base image
FROM python:3.9-slim

# Set the working directory inside the container
WORKDIR /app

# Copy only the application file
COPY app.py /app/

# Install Flask directly
RUN pip install --no-cache-dir flask

# Expose the Flask default port
EXPOSE 5000

# Optional environment variable
ENV NAME="Docker"

# Run the application
CMD ["python", "app.py"]

🏗️ Build the Docker Image

From inside your project directory (my-docker-app), run:

docker build -t my-flask-app .

💡 Explanation:

• docker build: Command to build an image.
• -t my-flask-app: Tags the image with a name (my-flask-app).
• .: Tells Docker to use the current directory as the build context (where it looks for files like Dockerfile, app.py, etc.).

You’ll see output showing each step being executed — downloading Python, installing Flask, copying files, etc.

▶️ Run the Container

Once the image is built, run it as a container:

docker run -d -p 5000:5000 my-flask-app

💡 Explanation:

• -d: Run the container in detached mode (in background).
• -p 5000:5000: Map port 5000 on your host to port 5000 in the container.
• my-flask-app: The image to run.

🌍 Test the App

Open your browser and go to:

👉 http://localhost:5000

You should see:

Hello from Docker!

🎉 You’ve successfully built a Docker image and run a web app inside a container!

🗑️ Optional: Clean Up

To stop and remove the container:

docker stop <container_id>
docker rm <container_id>

Use docker ps to find the container ID.

📋 Summary of Key Commands

✅ Bonus Tip: Best Practices

• Always use .dockerignore to exclude unnecessary files (like __pycache__, .git, venv, etc.)
• Avoid installing unnecessary dependencies
• Use multi-stage builds for more complex apps (advanced topic)

Push the docker image to Docker Hub

🔐 Log in to Docker Hub

In your terminal, run:

docker login

You’ll be prompted to enter your Docker Hub username and password.

Example:

Username: your-dockerhub-username
Password: **********
Login Succeeded

✅ If successful, you’re now logged in and ready to push images.

🏗️ Tag Your Image with Docker Hub Username

Before pushing an image to Docker Hub, you need to tag it with your Docker Hub username, like this:

docker tag my-flask-app your-dockerhub-username/my-flask-app

Replace your-dockerhub-username with your actual Docker Hub username.

💡 Explanation:

• docker tag: Renames or tags an existing image.
• my-flask-app: The local image name you built earlier.
• your-dockerhub-username/my-flask-app: The new name for Docker Hub.

You can verify the tagging by running:

docker images

You should see both my-flask-app and your-dockerhub-username/my-flask-app listed.

🚀 Push the Image to Docker Hub

Now push your image:

docker push your-dockerhub-username/my-flask-app

This uploads the image to Docker Hub.

Example Output:

The push refers to repository [docker.io/your-dockerhub-username/my-flask-app]
...
Pushed
latest: digest: sha256:... size: ...

✅ Success! Your image is now available on Docker Hub.

🌍 Test Pulling Your Image from Docker Hub

To confirm everything works, stop and remove your local image:

docker stop $(docker ps -aq)
docker rm $(docker ps -aq)
docker rmi your-dockerhub-username/my-flask-app
docker rmi my-flask-app

Then pull it back from Docker Hub:

docker pull your-dockerhub-username/my-flask-app

And run it:

docker run -d -p 5000:5000 your-dockerhub-username/my-flask-app

Go to http://localhost:5000 — you should still see:

Hello from Docker!

🎉 You’ve successfully pushed your image to Docker Hub and pulled it again!

📋 Summary of Key Commands

📝 Optional: Add a Tag (e.g., Version)

Instead of always using latest, you can version your image:

docker tag my-flask-app your-dockerhub-username/my-flask-app:v1.0
docker push your-dockerhub-username/my-flask-app:v1.0

Then pull with:

docker pull your-dockerhub-username/my-flask-app:v1.0

🧼 Bonus: Clean Up Local Images

To free up space:

docker images prune -a

Or just remove specific images:

docker rmi your-dockerhub-username/my-flask-app