yes, it is from the swift-unidoc project (guide).
i wouldn’t try to run the UnidocServer, it is comparatively a lot of effort to set up (but if you want, you can follow the steps in the guide.)
probably if i were you, i’d try running the UnidocBuild tool instead, which is easier to test and exhibits the same problem. it comes with a docker image.
the easiest way to trigger a fatal error is to just invoke the command without any of its required arguments.
$ docker run -it --rm --entrypoint=/bin/bash tayloraswift/unidoc:latest
root@bbce10ca8dc8:/# unidoc-build
UnidocBuild/Main.Options.swift:43: Fatal error: Usage: unidoc-build <package>