hbz Central IT hosts virtualization clusters on Citrix Xen Server 6.1.0 .
Installed server 'folio-dev.hbz-nrw.de' with SLES12.3
Virtualbox doens't run in a Xen environment => don't work with virtualbox, install directly on virtual server folio-dev . Create Docker containers on that server.
8 GB RAM of that server might be a bit too low for a production environment (should be 16 GB RAM).