Why registries store and distribute images in Docker - Why It Works

Introduction

Docker registries keep copies of container images so you can share and reuse them easily. They solve the problem of moving images between computers and teams without rebuilding every time.

When you want to share your app image with teammates or other servers.

When you need to deploy the same container image on multiple machines.

When you want to save storage by reusing images instead of rebuilding.

When you want to keep a backup of your container images in a central place.

When you want to use public images from others without downloading them manually.

Commands

This command downloads the nginx image version 1.23.3 from Docker Hub, a public registry. It shows how registries distribute images to your local machine.

Terminal

docker pull nginx:1.23.3

Expected Output

1.23.3: Pulling from library/nginx

Digest: sha256:abc123def456...

Status: Downloaded newer image for nginx:1.23.3

docker.io/library/nginx:1.23.3

This command lists all images stored locally on your machine, including the one just pulled from the registry.

Terminal

docker images

Expected Output

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE

nginx        1.23.3    123abc456def   2 weeks ago   133MB

This tags the nginx image with a new name to prepare it for uploading to your own registry or Docker Hub under your username.

Terminal

docker tag nginx:1.23.3 myusername/myapp:latest

Expected Output

No output (command runs silently)

This uploads your tagged image to the Docker Hub registry so others or your servers can download it later.

Terminal

docker push myusername/myapp:latest

Expected Output

The push refers to repository [docker.io/myusername/myapp]

123abc456def: Pushed 

latest: digest: sha256:789xyz123abc size: 1234

Key Concept

Registries store container images centrally so you can easily share, reuse, and deploy them anywhere without rebuilding.

Common Mistakes

Trying to push an image without tagging it with your registry username.

Docker requires images to be tagged with the registry path before pushing, or it will fail.

Always tag your image with your username or registry path before pushing, e.g., docker tag nginx:1.23.3 myusername/myapp.

Pulling an image without specifying the correct tag.

Docker pulls the 'latest' tag by default, which may not be the version you want.

Specify the exact tag like nginx:1.23.3 to get the correct image version.

Summary

Docker registries keep container images so you can share and reuse them easily.

You pull images from registries to run containers locally or on servers.

You push your own images to registries after tagging them with your username.