At some point in the last weeks the composer docker image that can be found on docker hub switched from PHP7 to PHP8. Some projects, such as wikibase-docker, still require PHP7 for installation.
It looks like the composer image is not going to be providing an additional tag using a pre PHP 8 version, thus an old digest will be needed.
tldr; here is the digest that you want for composer with PHP7 (tag-details)
docker pull composer@sha256:d374b2e1f715621e9d9929575d6b35b11cf4a6dc237d4a08f2e6d1611f534675
Figuring out the digest
All of the docker images provided on docker hub in the _ namespaces, exist in an official-images github repository alongside an extra repo-info github repository with some extra information of builds.
The repo-info repository contains a whole bunch of extra information for the docker images that are created and pushed to docker hub.
Everything to do with the composer image can be found here.
tag-details.md file contains output of the tags that were pushed to docker hub as part of each commit. This includes the steps in each image, as well as the digest of the resulting image.
Jumping through the history of that file you can easily find the version that last included
php-7 in one of the steps (here).
The digest listed there can then continue to be used in projects needing PHP 7 composer support in docker.
Thes same process can be used for any docker image in the docker image library on docker hub.