Here is how I build now a lot of unofficial builds without error :
The last Debian without any desktop environment (because I have only 6 GB of RAM and my laptop is freezing with desktop environment, even light ones), so only a terminal.
I don’t use docker but the manual method, based on a script I made but you can use the one from Harvey, the 161.sh script (they are quite similar).
And if you have issues… Read all the not digest topics to find solutions 
PS : I told I would make a HOWTO for newbies and that’s still the plan but I have also other things to do. One day it will come.
Do not hesitate to create a topic for the build you want to make and ask for help.