Images and registries

In plain English

A registry is a remote library that stores and shares images, and Docker Hub is the public registry Docker checks by default. An image tag is a label that names a version, so nginx:1.27 and nginx:alpine point at different builds of the same image. docker pull downloads an image from a registry, and docker push uploads one you built, after you log in with docker login. Tags such as latest are mutable, meaning the same tag can silently point at a new image later, so pin a specific version or a digest for repeatable builds. An image is made of stacked layers, so choosing a small base image and installing fewer packages keeps pull times and disk usage low.

Step by step

1. 1Log in to Docker Hub with docker login.
2. 2Pull an image by tag with docker pull node:22-alpine.
3. 3Tag a local image for your Docker Hub account with docker tag.
4. 4Upload the image with docker push.
5. 5Compare image sizes with docker images.

docker login
docker pull node:22-alpine
docker tag node:22-alpine myname/my-app:1.0
docker push myname/my-app:1.0
docker images

Common mistakes

• Deploying latest means a later pull can bring different code than the one you tested.
• Pushing to Docker Hub without logging in first fails with a permission error.
• Assuming a larger image is better is wrong; extra packages slow pulls and widen the attack surface.
• Forgetting to tag an image before push fails, because Docker needs a registry and repository name.