Thanks for your responses. You first stated that " The reason for the 404 page is that lighttpd doesn’t know how to answer the request", and then that I could "tail the lighttpd access log" to "see why lighttpd has decided to return this page". Didn't you just answer that, or did I misunderstand?
Ultimately, my goal is to write up a deployment and maintenance document so people who are not me can spin the VM/Docker/PiHole thing up as-needed, and the end-goal is to have as much configuration done in the webui as possible. Extra config modification for things under the hood is just asking for my techs to have issues. http://pihole.domain.local/admin/ works fine, it's http://pihole.domain.local/ that returns a confusing message, and it's probably easier for me to just include in the document "don't go there; go here instead" rather than debate it until it's smooth.