[FROG] Where is ./bootstrap.sh coming from?

Hi I am tyring to build a Docker image for FRR using https://github.com/FRRouting/frr/blob/master/docker/ubuntu20-ci/Dockerfile. When it gets to line 55, it fails on ./bootstrap/.sh. I have manually inspected the Dockerfile and it seems that the only thing that would write anything to /home/frr/frr is the libyang build. But that isn't producing any file by that name. I tried manually building libyang, too. Where is bootstrap.sh supposed to come from? Thanks and best regards -- Saqib Ilyas