Docker Course Labs

Accessing Images on Registries

Registries are servers which store Docker images. Docker Hub is the most popular, but the registry API is standard and you can run your own registry in the cloud or locally as your private image store.

Organizations run their own registries so they can have a long-term store of release versions from the build pipeline, or to make images available in the same network region as the production environment.

Reference

CLI overview

It's still image commands to work with registries. The most popular commands have aliases:

docker image --help

docker pull --help

docker push --help

docker tag --help

Pushing images

Any images you build are only stored on your machine.

To share images you need to push them to a registry - like Docker Hub. For Docker Hub the image name needs to include your username, which Docker Hub uses to identify ownership.

Make sure you've registered on Docker Hub. Then save your Docker ID in a variable, so we can use it in later commands:

# on Linux or macOS:
dockerId='<your-docker-hub-id>'

# OR with PowerShell:
$dockerId='<your-docker-hub-id>'

This is your Hub username not your email address. For mine I use: $dockerId='sixeyed'

Now you can build an image, including your Docker ID in the name:

docker build -t ${dockerId}/curl:21.06 -f labs/images/curl/Dockerfile.v2 labs/images/curl

docker image ls '*/curl'

📋 Now push your own curl:21.06 image to Docker Hub.

Not sure how?
# log in if you haven't already:
docker login -u ${dockerId}

# push your image:
docker push ${dockerId}/curl:21.06

Docker Hub images are publicly available (you can create private images too). Run this command and browse to your image on Docker Hub:

echo "https://hub.docker.com/r/${dockerId}/curl/tags"

Tags and references

Image names (properly called references) are built from three parts:

A single image can have multiple references, which are like aliases - they're not copies of the image, they're different names for the same image.

Tags are typically used for versioning, so you can assume that sixeyed/curl:20.09 and sixeyed/curl:21.06 are different versions of the same app, published by the same owner.

Add a new reference for your image using the tag command:

docker tag ${dockerId}/curl:21.06 ${dockerId}/curl:21.07

docker image ls '*/curl'

Now you have lots of curl images, but all the aliases have the same image ID

📋 Push all of your curl image tags to Docker Hub.

Not sure how?
# you can push individual tags:
docker push ${dockerId}/curl:21.07

# or all local tags for the repository:
docker push --all-tags ${dockerId}/curl

You'll see lots of Layer already exists output - registries have the same layer caching approach as the Docker Engine.

Running a local registry

Docker images for real apps can be big - hundreds of megabytes or even gigabytes. Downloading large images adds to the startup time for containers, so organizations typically run their own registry in the cloud or the datacenter.

All registries work in the same way, so they're pretty much interchangeable. You'd use Azure Container Registry or Amazon Elastic Container Registry if you were running on Azure or AWS.

In a local environment you can host your own registry, running in a container, using the official Docker Registry image:

docker run -d -p 5000:5000 --name registry registry:2.7.1

docker logs registry

Your local registry domain is localhost:5000 so you can include that in image references to push and pull locally.

The full image reference format is:

📋 Tag the Alpine image and push it to your local registry.

Not sure how?
# the tag command creates an alias, which can include the registry domain:
docker tag alpine localhost:5000/alpine

# pushing a tag with a domain in the reference tells Docker which registry to use:
docker push localhost:5000/alpine

Docker Desktop is configured to allow localhost registries, but not all Docker setups have that. If you get an error when you push, you need to allow insecure registries.

The open-source registry doesn't have a web UI like Docker Hub, but you can work with it using the REST API:

curl --head localhost:5000/v2/

curl localhost:5000/v2/alpine/tags/list

Every image has a tag - if you don't supply one, Docker uses a default.

You can remove your local image and pull it again from the registry:

docker image rm localhost:5000/alpine

docker pull localhost:5000/alpine

📋 Create and push some more tags for the Alpine image - check they're available in the registry.

Not sure how?
# you can use any string in the image tag:
docker tag alpine localhost:5000/alpine:21.07

docker tag alpine localhost:5000/alpine:local

docker push --all-tags localhost:5000/alpine

curl localhost:5000/v2/alpine/tags/list

You should see the default tag and your new tags returned.

Lab

Image references have enough detail for Docker to find the registry, and identify a specific version of the application.

Docker uses defaults for the registry and the tag. What are those defaults? What is the full reference for the image kiamol/ch05-pi?

Not all official images are on Docker Hub. Microsoft uses its own image registry MCR at mcr.microsoft.com. What command would you use to pull version 5.0 of the dotnet/runtime image from MCR?

Stuck? Try hints or check the solution.


Cleanup

Cleanup by removing all containers:

docker rm -f $(docker ps -aq)