ReadList #2 - Containerizing an Application:

1. How to Containerize an Application?

Containerizing means packaging an app with its dependencies so it runs consistently across different environments.
Steps:

1. Write a Dockerfile → Define the container's environment.

2. Build the Image → Convert the Dockerfile into a runnable image.

3. Push the Image → Store it in a registry for easy access.

4. Pull & Run → Deploy it in Dev, Test, Prod environments.

Example:

FROM node:18  
WORKDIR /app  
COPY . .  
RUN npm install  
CMD ["node", "server.js"]

• This Dockerfile defines how to package a Node.js app.

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2. Layers in a Docker Image

Each command in the Dockerfile creates a new layer.

• Why Layers?

  • Caching → Speeds up rebuilds.

  • Efficient Storage → Reuses unchanged layers.

  • Layered File System → Read-only base + writeable container layer.

• Example:

    FROM python:3.9   # Layer 1
    COPY . .          # Layer 2  
    RUN pip install -r requirements.txt  # Layer 3
  

  • If code changes but dependencies don’t, Layer 3 is reused.

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3. Who Writes Dockerfiles?

• Developers (Dev) → Write Dockerfiles for their applications (even for production).

• DevOps Engineers →

  • Optimize Dockerfiles.

  • Handle environment-specific configurations (e.g., networking, scaling).

  • Secure and manage images.

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4. Docker Commands

Building a Docker Image

docker build -t myapp .

• -t myapp → Tags the image as myapp.

Pushing an Image to Docker Hub

docker tag myapp username/myapp:v1  
docker push username/myapp:v1

• Tagging is important! It helps version control.

Logging into Docker Hub

docker login

• Prompts for username/password.

Viewing Local Docker Images

docker images

• Shows all available images on the system.

Pulling an Image from Docker Hub

docker pull nginx:latest

• Pulls the latest Nginx image.

Running a Container

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

• -d → Runs in detached mode.

• --name mycontainer → Names the container.

• -p 8080:80 → Maps host port 8080 to container port 80.

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5. Container Name & ID

• Container Name:

  • Custom: docker run --name mycontainer ubuntu

  • Auto-generated: If not provided, Docker assigns a random name.

• Container ID:

  • Unique identifier for each container.

  • Get running containers:

      docker ps
    

  • Stop a container using ID:

      docker stop <container_id>
    

Conclusion:
In this session, we covered the essential steps to containerize an application, from writing a Dockerfile to building, pushing, and running Docker images. We also explored the concept of layers in Docker images, the roles of developers and DevOps engineers in managing Dockerfiles, and some fundamental Docker commands for building, pushing, and running containers. Understanding these concepts is crucial for efficiently deploying applications across different environments. Stay tuned for more practical insights in the next ReadList session!