Nested virtualisation. To my knowledge VirtualBox does not yet support this, and has been steadfast in its refusal for years[1].<p>[1] <a href="https:&#x2F;&#x2F;www.virtualbox.org&#x2F;ticket&#x2F;4032" rel="nofollow">https:&#x2F;&#x2F;www.virtualbox.org&#x2F;ticket&#x2F;4032</a>