Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How are images downloaded?

• How are images distinguished?

Objectives

• Pull images from Docker Hub image registry

• List local images

• Introduce image tags

Docker Hub

Much like how GitHub allows for web hosting and searching for code, the Docker Hub image registry allows the same for Docker images. Hosting images is free for public repositories and allows for downloading images as they are needed.

Additionally, through integrations with GitHub and Bitbucket, Docker Hub repositories can be linked against Git repositories so that automated builds of Dockerfiles on Docker Hub will be triggered by pushes to repositories. However, at this moment enabling such a feature requires a Pro (paid) account or joining the Docker-Sponsored Open Source Program. There are other ways of doing this, such as using GitLab/GitHub CI/CD, but that’s beyond the scope of this training module.

Docker Hub and Podman

Both Docker and Podman use OCI (Open Container Initiative) compliant images, so you can use the same images with both tools. It means Podman can pull and run images from Docker Hub.

By default, podman pull pulls an image from Docker Hub if a registry is not specified in the command line argument.

Pulling Images

To begin with we’re going to pull down the image we’re going to be working in for the tutorial (note: if you did all the docker pulls in the setup instructions, this image will already be on your machine, in which case podman should notice it’s there and not attempt to re-pull it unless it’s changed in the meantime):

podman pull matthewfeickert/intro-to-docker

No search registry defined

Some installations of Podman may end with an error like Error: unable to pull matthewfeickert/intro-to-docker:latest: unable to find registry in the system. This is because the default registry is not defined. You can fix this by adding the docker.io registry at the command:

podman pull docker.io/matthewfeickert/intro-to-docker

Or, to pull images by default from Docker Hub, adding the following line to the /etc/containers/registries.conf file:

unqualified-search-registries=["docker.io"]

Connection errors

If using Podman or Docker in a non-Linux machine you run into an error like Error: unable to connect to Podman, make sure that the Podman or Docker desktop application is running.

Remember that in such environments, Podman or Docker use a virtual machine to run the containers.

and then list the images that we have available to us locally

podman images

If you have many images and want to get information on a particular one you can apply a filter, such as the repository name

podman images matthewfeickert/intro-to-docker

REPOSITORY                        TAG                 IMAGE ID            CREATED             SIZE
matthewfeickert/intro-to-docker   latest              cf6508749ee0        3 months ago        1.49GB

or more explicitly

podman images --filter=reference="matthewfeickert/intro-to-docker"

REPOSITORY                        TAG                 IMAGE ID            CREATED             SIZE
matthewfeickert/intro-to-docker   latest              cf6508749ee0        3 months ago        1.49GB

You can see here that there is the TAG field associated with the matthewfeickert/intro-to-docker image. Tags are a way of further specifying different versions of the same image. As an example, let’s pull the buster release tag of the Debian image (again, if it was already pulled during setup, podman won’t attempt to re-pull it unless it’s changed since last pulled).

podman pull debian:buster
podman images debian

buster: Pulling from library/debian
<some numbers>: Pull complete
Digest: sha256:<the relevant SHA hash>
Status: Downloaded newer image for debian:buster
docker.io/library/debian:buster

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
debian              buster              00bf7fdd8baf        5 weeks ago         114MB

Check the documentation on pull and images for more information on these commands.

Pulling Python

Pull the image python:3.9-slim for Python 3.9 and then list all python images on your computer.

Browse the official Python images to find available tags and read about image variants. What does -slim mean?

Solution

podman pull python:3.9-slim
podman images --filter=reference="python"

REPOSITORY                        TAG                 IMAGE ID            CREATED             SIZE
docker.io/library/python          3.9-slim            e440e2151380        2 weeks ago        131 MB

• python:<version>-slim: This image does not contain the common packages contained in the default tag and only contains the minimal packages needed to run python

Key Points

• Pull images with podman pull <image-id>

• List all images on the computer and other information with podman images

• Image tags distinguish releases or version and are appended to the image name with a colon