5. Images and Registries
Work with image names, tags, digests, pulls, and local metadata.
Image References
An image reference can include a registry, repository, and tag. For example, nginx:alpine identifies the nginx repository with the alpine tag on the default registry.
A tag is a movable label. A digest identifies specific image content. Use an approved digest when a release must reference an exact artifact.
Inspect an Image
docker pull nginx:alpine
docker image ls
docker image inspect nginx:alpine
docker image history nginx:alpine
The history helps explain how the image was assembled. It is not a substitute for scanning or reviewing the source Dockerfile.
Retag Locally
docker tag nginx:alpine docker-course-web:demo
docker image inspect docker-course-web:demo
Tagging creates another reference to the image; it does not rebuild it. Pushing requires a registry repository you can write to and appropriate authentication.
Common Mistakes
- Assuming
latestmeans the newest or most secure image. - Rebuilding separately in each deployment environment instead of promoting a tested artifact.
- Assuming an image supports every CPU architecture.
Practice
Compare the image IDs of the original and local tag. Explain why their names differ but their content is shared.
Quick Interview Answer
Registries distribute images. Tags are convenient labels, while digests identify content precisely.
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