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In this guide, you’ll learn how to:
  • Pull images from public and private registries
  • Authenticate with docker login
  • Retag (rename) images
  • Push images to private registries
  • Inspect actual disk usage with Docker

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1. Pulling Images from a Public Registry

Pulling from a public registry (for example, the official Ubuntu image on Docker Hub) requires no authentication:
By default, docker pull ubuntu retrieves the latest tag. To pull a specific version, append the tag (e.g., ubuntu:20.04).

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2. Accessing Private Repositories

Attempting to pull an image from a private registry without logging in will result in an access denied error:
Similarly, pushing without authentication will fail:
You must authenticate before pulling from or pushing to private registries.
Make sure your credentials have the necessary permissions.

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3. Logging In to a Registry

Use docker login to authenticate. By default, it targets Docker Hub (docker.io).
Example:
For other registries such as Google Container Registry:
Once logged in, subsequent docker pull and docker push commands will use your credentials.

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4. Retagging (Renaming) an Image

Docker images cannot be renamed directly. Instead, you can create a new tag—a “soft link” to the same image ID.
  1. List local images:
  2. Retag httpd:alpine to httpd:customv1:
  3. Confirm the new tag:
Example output:
Notice httpd:alpine and httpd:customv1 share the same IMAGE ID—no duplicate layers are created.

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5. Pushing to a Private Registry

To push your retagged image:
Make sure you are logged in (docker login gcr.io) before pushing.

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6. Checking Actual Disk Usage

Use docker system df to inspect disk usage across images, containers, volumes, and build cache:
Example:
Even if docker image ls lists four tags, docker system df shows only three unique images. The SIZE column reflects unique layers, and RECLAIMABLE indicates potential space savings.

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Command Reference Table

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