This dhcpd config requires a slightly special setup in order to boot servers using iPXE. http://ipxe.org/howto/chainloading contains an overview of what is needed. I prefer to use http://chschneider.eu/linux/server/tftpd-hpa.shtml since it's stupid easy to setup. None of the setup in the guide is needed unless you want to change something. After it's intalled drop your undionly.kpxe file in /var/lib/tftpboot.
To get rolling fill in the isc-dhcp-server file INTERFACE section with the interface that you would like to listen to traffic on. We are using host networking so we don't have to care about docker interfaces. If someone wants to make this run in bridge mode put up a PR.