When using an agent directive in Declarative Pipeline there is no way to specify the Docker Registry to use. In Scripted Pipline this is available via the withRegistry wrapper

docker.withRegistry('https://docker.mycorp.com/', 'docker-login') {
  git '…'
  docker.build('myapp').push('latest')
}

For Declarative we should allow admins to create a default registry to be used by all Pipelines at the master level or the folder level. We should also allow a specific registry setting in the language itself via the agent directive.