Day 03: Docker Images and Containers: Building, Pulling, and Running Docker Containers

With Docker installed, you’re ready to dive into the core of containerization—working with Docker images and containers. This article will guide you through building, pulling, and running containers, helping you understand their practical use in development and deployment.

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Key Concepts: Docker Images vs. Containers

Before we proceed, let’s clarify two important terms:

I. Docker Images:

• A Docker image is a lightweight, standalone, and executable package that includes everything needed to run a piece of software: code, runtime, libraries, and environment settings.
• Images are immutable and serve as blueprints for creating containers.

II. Docker Containers:

• A container is a runtime instance of a Docker image.
• It is isolated and portable, running the application as specified in the image.

Think of an image as a recipe and a container as the dish prepared from that recipe.

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Pulling a Docker Image

Docker Hub hosts thousands of pre-built images. To use an image, you first need to pull it. Here’s how:

Example: Pulling the NGINX Image

I. Open your terminal.
II. Run the following command:

   docker pull nginx

III. Docker will fetch the latest version of the NGINX image from Docker Hub.

To list all downloaded images, use:

docker images

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Running a Container

Once you have an image, you can create and run a container from it.

Example: Running an NGINX Container

I. Use the following command:

   docker run --name my-nginx -d -p 8080:80 nginx

• --name my-nginx: Names the container "my-nginx."
• -d: Runs the container in detached mode.
• -p 8080:80: Maps port 8080 on your host to port 80 in the container.

II. Open your web browser and navigate to http://localhost:8080. You should see the default NGINX welcome page.

To view running containers, use:

docker ps

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Building Your Own Docker Image

Sometimes, you’ll want to create a custom image tailored to your application. To do this, you’ll use a Dockerfile.

Example: Building a Simple Web Server Image

I. Create a Dockerfile:

   # Use an official Python runtime as a parent image
   FROM python:3.9-slim

   # Set the working directory in the container
   WORKDIR /app

   # Copy the current directory contents into the container
   COPY . /app

   # Install dependencies
   RUN pip install --no-cache-dir flask

   # Make port 5000 available to the world outside the container
   EXPOSE 5000

   # Define the command to run the application
   CMD ["python", "app.py"]

II. Place a simple Flask app (app.py) in the same directory:

   from flask import Flask

   app = Flask(__name__)

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

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

III. Build the image:

   docker build -t my-flask-app .

VI. Run the container:

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

Visit http://localhost:5000 to see your application in action.

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Managing Containers

Stopping and Removing Containers

• Stop a container:

  docker stop <container_id>

• Remove a container:

  docker rm <container_id>

Removing Images

• Remove an image:

  docker rmi <image_id>

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Conclusion

In this article, you’ve learned how to pull images, run containers, and even build your own custom image. These skills form the foundation of working with Docker.

In the next article, we’ll explore Docker Compose, a powerful tool for managing multi-container applications. Stay tuned!