fedora-image-uploader: Add the push cert for registry.fedoraproject.org
    
    To push images, we need to use a client certificate and key[0]. Add
    those to the image uploader container.
    
    [0] https://pagure.io/fedora-infra/ansible/blob/7a10d32e16a81b88c478e6ad4f734cbcc0e75b83/f/playbooks/groups/releng-compose.yml#_144