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