Few companies that used them that I dealt with had their own tooling, but that was nothing special.

CI picks up your image build, then commits its record to a DB from which DHCP server looks up a boot image for given MAC, then it either SSHed a reboot command or used an IPMI reboot. Voila

Today, some server motherboard firmwares can boot images off HTTP, vlan, and can talk with the boot server itself on HTTP

You could have if HashiCorp had beaten Docker to market with a simple VagrantHub and bud tool...

It could have easily been VMs instead of containers. IMHO, it was all the packaging...

~$ qemu diskimage.img