I have not consistently reproduced this but there seems to be a situation where the compute resource model is loaded after a plugin (docker) has registered itself as a provider. This results in it being missing from the list of supported_providers.

https://github.com/theforeman/foreman/blob/develop/app/models/compute_resource.rb#L13

  class_attribute :supported_providers
  self.supported_providers = {
    'Libvirt'   => 'Foreman::Model::Libvirt',
    'Ovirt'     => 'Foreman::Model::Ovirt',
    'EC2'       => 'Foreman::Model::EC2',
    'Vmware'    => 'Foreman::Model::Vmware',
    'Openstack' => 'Foreman::Model::Openstack',
    'Rackspace' => 'Foreman::Model::Rackspace',
    'GCE'       => 'Foreman::Model::GCE',
  }