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docker-explanation.md

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What is Docker in Simple Terms?

Imagine you want to run a program on your computer, but that program has many dependencies (other software, libraries, or configurations). Instead of manually setting up everything, Docker packages the program, its dependencies, and the environment into something called a container. A container is a lightweight, portable environment that runs the program exactly the same way, no matter where it's running—whether it’s on your computer, a server, or in the cloud.

Key idea: Docker lets you "containerize" applications, so they run smoothly in any environment without worrying about setup or configuration conflicts.

Basic Docker Concepts

  • Image: A blueprint or template for a container. It’s like a recipe for what should be in the container.
  • Container: A running instance of an image. It’s like the actual dish created from the recipe.
  • Dockerfile: A file that contains the instructions to build a Docker image.
  • Docker Hub: A repository (like GitHub for code) where Docker images can be stored and shared.
  • Azure Container Registry (ACR): A repository (like GitHub for code) where Docker images can be stored and shared but within the Azure ecosystem.

Exercises to Practice Docker Basics

Here are some exercises to get you familiar with Docker commands like push, pull, tag, build, and run.

1. Docker Pull

Exercise: Pull an existing image from Docker Hub.

docker pull hello-world
  • What happens: This command fetches the hello-world image from Docker Hub.
  • Purpose: Learn how to download (or "pull") an image from Docker Hub to your local machine.

2. Docker Run

Exercise: Run the hello-world container.

docker run hello-world
  • What happens: Docker will run the container, and you’ll see a simple message printed in the terminal.
  • Purpose: Understand how to start (or "run") a container using a downloaded image.

3. Docker Build

Exercise: Build your own Docker image using a simple Dockerfile.

  1. Create a Dockerfile with the following content:

    # Use the official Node.js image from Docker Hub
FROM node:14

# Set the working directory inside the container
WORKDIR /app

# Copy the local files to the container's working directory
COPY . .

# Run a command inside the container to install dependencies
RUN npm install

# Command to run your application
CMD ["node", "index.js"]

  2. Create a basic index.js file:

    console.log('Hello from Docker!');

  3. Build the image:

    docker build -t my-node-app .
    • What happens: This command builds a new Docker image from the Dockerfile and tags it as my-node-app.
    • Purpose: Learn how to create your own Docker image using a Dockerfile.

4. Docker Tag

Exercise: Tag your image with a different name or version.

docker tag my-node-app my-node-app:v1
  • What happens: This tags your my-node-app image with the version v1.
  • Purpose: Learn how to assign additional names or versions to your images.

5. Docker Push

Exercise: Push your image to Docker Hub (you’ll need a Docker Hub account).

  1. Log in to Docker Hub: 
    docker login
  2. Tag your image to match your Docker Hub repository: 
    docker tag my-node-app:v1 your-dockerhub-username/my-node-app:v1
  3. Push the image to Docker Hub: 
    docker push your-dockerhub-username/my-node-app:v1
    • What happens: The image is uploaded to Docker Hub under your repository.
    • Purpose: Learn how to upload (or "push") your Docker images to a public or private repository.

6. Docker Pull & Run Custom Image

Exercise: Pull the image you just pushed to Docker Hub and run it.

  1. Pull the image from Docker Hub: 
    docker pull your-dockerhub-username/my-node-app:v1
  2. Run the container: 
    docker run your-dockerhub-username/my-node-app:v1
  • What happens: The container will run, and you'll see the output "Hello from Docker!" in the terminal.
  • Purpose: Learn how to fetch and run your custom Docker images on any machine.

Summary of Docker Commands:

  • docker pull <image>: Download an image from Docker Hub.
  • docker run <image>: Run a container from the image.
  • docker build -t <name> <path>: Build a Docker image from a Dockerfile.
  • docker tag <image> <new-name>: Assign a new name or version to an image.
  • docker push <image>: Upload an image to Docker Hub.

Command line list of Docker Commands:

  • docker rm -f <imageid>: Remove an image.
  • docker images: List all images available local.
  • docker rm -vf $(docker ps -aq): Remove all containers, also the ones running.
  • docker rmi -f $(docker images -aq): Remove all images.