i probably wasn’t clear enough, ld.gold is usable for building the toolchain itself, but this toolchain will not be able to use ld.gold to compile a project of sufficient complexity. instead, you must use the ld linker. one way to do this is to pass Xlinker flags when building your application. but anecdotally, symlinking /usr/bin/ld.gold to /bin/ld also worked for me.
i was also confused by the layout of the swift-docker repository.