Imagine a library where books are catalogued, stored, and borrowed by readers. In DevOps, Docker registries are that library — they store Docker images, catalog them with tags, and make them available for teams and environments to pull and run. Without registries, CI/CD pipelines would have no central hub to distribute images.

Registry Foundations

1. What is a Docker Registry?

• A Docker registry is a centralized service for storing and distributing Docker images.
• Images are tagged and versioned, making them easy to manage.
• Registries enable collaboration across teams and environments.

2. Types of Registries

• Docker Hub: Default public registry.
• Private Registries: Self‑hosted or cloud‑hosted (e.g., Harbor, Nexus).
• Cloud Registries:
  • AWS Elastic Container Registry (ECR).
  • Azure Container Registry (ACR).
  • Google Artifact Registry / Container Registry.

3. Role of Registries in CI/CD

• CI Stage: Builds and tests images.
• Registry Stage: Pushes validated images to registry.
• CD Stage: Pulls images from registry for deployment.
• Registries act as the bridge between CI and CD.

4. Registry Best Practices

• Use version tags (e.g., myapp:v1.2.0) instead of latest.
• Secure registries with authentication and TLS.
• Automate cleanup of unused images.
• Integrate registries with CI/CD tools for seamless pipelines.

Things to Remember

• Registries are the distribution hub of CI/CD pipelines.
• Public registries are easy to use, but private/cloud registries offer more control.
• Tagging and security are critical for production reliability.

Hands‑On Lab

Step 1: Login to Docker Hub

docker login -u myusername -p mypassword

Step 2: Tag Image

docker tag myapp:latest myusername/myapp:v1.0.0

Step 3: Push Image to Registry

docker push myusername/myapp:v1.0.0

Step 4: Pull Image from Registry

docker pull myusername/myapp:v1.0.0

Step 5: Deploy Image from Registry

docker service create --name myapp --replicas 3 myusername/myapp:v1.0.0

Practice Exercise

1. Create a Docker image for a sample app.
2. Tag the image with a version (v1.0.0).
3. Push the image to Docker Hub.
4. Pull the image on another machine.
5. Deploy the image in a Swarm cluster.
6. Reflect on how registries enable collaboration and automation.

Visual Learning Model

CI/CD with Registries
   ├── CI → build & test image
   ├── Registry → push validated image
   ├── CD → pull image
   └── Deploy → Swarm/Kubernetes/Cloud

The Hackers Notebook

Docker registries are the backbone of CI/CD pipelines, storing and distributing images across environments. They act as the bridge between CI (build/test) and CD (deploy). By using proper tagging, authentication, and integration, registries ensure reliable, secure, and scalable deployments.