Use docker images from private registry

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.