Docker Lab, Excercise & Assignment – 5 – Docker Registry

Lesson Objectives

• Understanding where images come from
• Pulling a Docker image from Docker Hub
• Pushing a Docker image to Docker Hub

Getting an image from Docker Hub

Docker Hub is the place where open Docker images are stored. When we ran our first image by typing

docker run --rm -p 8787:8787 rocker/verse

the software first checked if this image is available on your computer and since it wasn’t it downloaded the image from Docker Hub. So getting an image from Docker Hub works sort of automatically. If you just want to pull the image but not run it, you can also do

docker pull rocker/verse

Getting an image to Docker Hub

Imagine you made your own Docker image and would like to share it with the world you can sign up for an account on https://hub.docker.com/. After verifying your email you are ready to go and upload your first docker image.

1. Log in on https://hub.docker.com/
2. Click on Create Repository.
3. Choose a name (e.g. verse_gapminder) and a description for your repository and click Create.
4. Log into the Docker Hub from the command linedocker login --username=yourhubusername --email=youremail@company.comjust with your own user name and email that you used for the account. Enter your password when prompted. If everything worked you will get a message similar toWARNING: login credentials saved in /home/username/.docker/config.json Login Succeeded
5. Check the image ID usingdocker imagesand what you will see will be similar to

REPOSITORY              TAG       IMAGE ID         CREATED           SIZE
verse_gapminder_gsl     latest    023ab91c6291     3 minutes ago     1.975 GB
verse_gapminder         latest    bb38976d03cf     13 minutes ago    1.955 GB
rocker/verse            latest    0168d115f220     3 days ago        1.954 GB

and tag your image

docker tag bb38976d03cf yourhubusername/verse_gapminder:firsttry

The number must match the image ID and :firsttry is the tag. In general, a good choice for a tag is something that will help you understand what this container should be used in conjunction with, or what it represents. If this container contains the analysis for a paper, consider using that paper’s DOI or journal-issued serial number; if it’s meant for use with a particular version of a code or data version control repo, that’s a good choice too – whatever will help you understand what this particular image is intended for.

6. Push your image to the repository you createddocker push yourhubusername/verse_gapminder

Your image is now available for everyone to use.

Saving and loading images

Pushing to Docker Hub is great, but it does have some disadvantages:

1. Bandwidth – many ISPs have much lower upload bandwidth than download bandwidth.
2. Unless you’re paying extra for the private repositories, pushing equals publishing.
3. When working on some clusters, each time you launch a job that uses a Docker container it pulls the container from Docker Hub, and if you are running many jobs, this can be really slow.

Solutions to these problems can be to save the Docker container locally as a a tar archive, and then you can easily load that to an image when needed.

To save a Docker image after you have pulled, committed or built it you use the docker save command. For example, lets save a local copy of the verse_gapminder docker image we made:

docker save verse_gapminder > verse_gapminder.tarCode language: CSS (css)

If we want to load that Docker container from the archived tar file in the future, we can use the docker load command:

docker load --input verse_gapminder.tarCode language: CSS (css)

You need to tag your image correctly first with your registryhost:

docker tag [OPTIONS] IMAGE[:TAG] [REGISTRYHOST/][USERNAME/]NAME[:TAG]  

Then docker push using that same tag.

docker push NAME[:TAG]  

Example:

docker tag 518a41981a6a myRegistry.com/myImage  docker push myRegistry.com/myImage

Challenge Questions

• Download your partner’s image. How did that compare speed-wise to downloading the verse image the first time?
• Browse Docker Hub for interesting images. What could be useful to you?
• Discuss the pros and cons of using Docker images from someone you don’t know with your neighbour.

Rajesh Kumar

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