Pod data-dir-etcd is failing with ImagePullBackOff in OpenShift 4

Issue

• What are the data-dir-etcd pods failing in OpenShift 4 cluster?

• There are data-dir-etcd pods failing with ErrImagePull and ImagePullBackOff in OpenShift 4 with messages like the following ones:

  Failed to pull image "etcd:3.5.10-0": reading manifest 3.5.10-0 in docker.io/library/etcd: requested access to the resource is denied