DockerHub recently updated certain images in the registry, making them OCI-compliant. Due to this change, attempts to pull some images results in the following error:
Error response from daemon: manifest for $ART_HOST/docker/ubuntu:20.04 not found: manifest unknown: The named manifest is not known to the registry.
This update mostly affects Artifactory versions 6.x and lower than 7.11.1. The reason for this is that Artifactory 6.x is not completely OCI compliant, and OCI image support has been added in Artifactory version 7.11.1 (see release notes).
Images that are OCI-compliant will include the following mediaType in the manifest file:
"mediaType": "application/vnd.oci.image.manifest.v1+json"
It is strongly advised to upgrade Artifactory to one of the supported 7.x versions in order to make use of OCI image support and be able to deploy and resolve OCI images in Docker registries.