Build process cannot pull image from the external registry which has a custom certificate

Issue

• I cannot deploy an image because building image fails while trying to pull the image from the external registry, which has a custom certificate:

  Cloning "https://github.com/example-app" ...
      Commit: 
      Author:
      Date:
  pulling image error : unknown: unable to pull manifest from docker-registry.example.com/rhscl/httpd-24-rhel7:latest: Get https://docker-registry.example.com/v2/:  x509: certificate signed by unknown authority
  error: build error: unable to get docker-registry.default.svc:5000/openshift/httpd@sha256:
  

• I can pull the image with docker or inspect with skopeo from the nodes, but it fails during deployment.

• External registry CA certificate is not trusted, but I have already copied it to the master.

NOTE: For OCP4 related issues please check on this documentation.