It takes a bit of know-how, but Samba running on just about any Linux distro can be setup to emulate a Windows domain controller. I’ve run this in the past and it’s pretty slick. You can even use the Windows RSAT tools to manage it. Though I don’t know that you would get the DHCP and DNS integration as you would with a Windows domain controller. Because these alternatives are niche I think most people would agree with me in saying “try these alternatives at your own risk because if it breaks you may be on your own without much in the way of help/support”. Worth looking into if nothing else.
https://wiki.samba.org/index.php/Setting_up_Samba_as_an_Active_Directory_Domain_Controller