Somebody reported the same thing on Github earlier - it was an oversight in the documentation:
https://github.com/pi-hole/docker-pi-hole/issues/1277
There is also now a note on the top of the Readme:
https://github.com/pi-hole/docker-pi-hole#upgrade-notes
Edit:
TL;DR:
if you have any modifications for lighttpd via an
external.conffile, this file now needs to be mapped into/etc/lighttpd/conf-enabled/whateverfile.confinstead