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 and Image Registries

Much like GitHub allows for web hosting and searching for code, the Docker Hub image registry allows the same for Docker images. Hosting and building of 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.

While Docker Hub is maintained by Docker and is the defacto default registry for Docker images, it is not the only registry in existence. There are many registries, both private and public, in existence. For example, the GitLab software allows for a registry service to be setup alongside its Git and CI/CD management software. CERN’s GitLab instance has such a registry available. See later episodes for more information on CERN’s GitLab Docker image registry. GitHub also now provides its own container registry called GHCR, which uses the namespace https://ghcr.io.

Pulling Images

To begin with we’re going to pull down the Docker 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 this case, docker should notice it’s there and not attempt to re-pull it, unless the image has changed in the meantime.):

docker pull sl

# if you run into a permission error, use "sudo docker run ..." as a quick fix
# to fix this for the future, see https://docs.docker.com/install/linux/linux-postinstall/

Using default tag: latest
latest: Pulling from library/sl
be7dd8a3f6cc: Pull complete
Digest: sha256:d20a8476d2369be2f3553382c9cce22f1aace2804cf52450b9dbacc93ae88012
Status: Downloaded newer image for sl:latest
docker.io/library/sl:latest

The image names are composed of NAME[:TAG|@DIGEST], where the NAME is composed of REGISTRY-URL/NAMESPACE/IMAGE and is often referred to as a repository. Here are some things to know about specifying the image:

• Some repositories will include a USERNAME as part of the image name (i.e. fnallpc/fnallpc-docker), and others, usually Docker verified content, will include only a single name (i.e. sl).
• A registry path (REGISTRY-URL/NAMESPACE) is similar to a URL, but does not contain a protocol specifier (https://). Docker uses the https:// protocol to communicate with a registry, unless the registry is allowed to be accessed over an insecure connection. Registry credentials are managed by docker login. If no registry path is given, the docker daemon assumes you meant to pull from Docker Hub and automatically appends docker.io/library to the beginning of the image name.
• If no tag is provided, Docker Engine uses the :latest tag as a default.
• The SHA256 DIGEST is much like a Git hash, where it allows you to pull a specific version of an image.
• CERN GitLab’s repository path is gitlab-registry.cern.ch/<username>/<repository>/<image_name>[:<tag>|@<digest>].

Now, let’s list the images that we have available to us locally

docker 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

docker images sl

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
sl           latest    33957a339e91   2 weeks ago   187MB

or more explicitly

docker images --filter=reference="sl"

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
sl           latest    33957a339e91   2 weeks ago   187MB

You can see here that there is the TAG field associated with the sl image. Tags are way of further specifying different versions of the same image. As an example, let’s pull the 7 release tag of the sl image (again, if it was already pulled during setup, docker won’t attempt to re-pull it unless it’s changed since last pulled).

docker pull sl:7
docker images sl

7: Pulling from library/sl
Digest: sha256:d20a8476d2369be2f3553382c9cce22f1aace2804cf52450b9dbacc93ae88012
Status: Downloaded newer image for sl:7
docker.io/library/sl:7

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
sl           7         33957a339e91   2 weeks ago   187MB
sl           latest    33957a339e91   2 weeks ago   187MB

Pulling Python

Pull the image python:3.7-slim for Python 3.7 and then list all python images along with the sl:7 image

Solution

docker pull python:3.7-slim
docker images --filter=reference="sl" --filter=reference="python"

3.7-slim: Pulling from library/python
42c077c10790: Pull complete
f63e77b7563a: Pull complete
dca49bd08fde: Pull complete
51a05345c44d: Pull complete
e69ebd661d90: Pull complete
Digest: sha256:f61a4c6266a902630324fc10814b1109b3f91ac86dfb25fa3fa77496e62f96f5
Status: Downloaded newer image for python:3.7-slim
docker.io/library/python:3.7-slim

REPOSITORY   TAG        IMAGE ID       CREATED       SIZE
python       3.7-slim   600bb8fe36b6   2 weeks ago   123MB
sl           7          33957a339e91   2 weeks ago   187MB
sl           latest     33957a339e91   2 weeks ago   187MB

Key Points

• Pull images with docker pull

• List images with docker images

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

• The default registry is Docker Hub