No longer an issue
Appears I spoke too soon. Dashboard isn't serving anything up externally and is only reflecting internal queries from the container itself.
docker network config:
pihole container config:
$ nslookup github.com 192.168.1.250
;; connection timed out; no servers could be reached
furthermore, when the container boots, it gets errors itself trying to resolve DNS:
If I change the container to bind to a bridge network, I get no DNS errors from within the container, but still cannot use it on the IP address I want to from outside:
$ nslookup github.com 192.168.1.250
;; connection timed out; no servers could be reached