TFTP

With Alpine Linux

# apk update
apk add tftp-hpa

After that add pxe files in /var/tftpboot/

Example alpine's boot.ipxe file check https://wiki.alpinelinux.org/wiki/Netboot_Alpine_Linux_using_iPXE

And start the tftp server

rc-update add in.tftpd default
rc-service in.tftpd start

Now you can use with your dhcpd server with showing next-server address.