x509: certificate signed by unknown authority pulling images from external registry in OpenShift 4

Issue

• Images fail to pull with the following error message shown in the events:

  Failed to pull image "registry.redhat.io/redhat/xxxxxx": rpc error: code = Unknown desc = pinging container registry registry.redhat.io: Get "https://registry.redhat.io/v2/": tls: failed to verify certificate: x509: certificate signed by unknown authority
  

• The Proxy CA is configured in the proxy resource of the cluster, and a curl to the registry via the Proxy is working properly, but the images are not pulled with above message.