You need to supply your own configuration, since znc lets you edit it on the fly; /.znc is a volume where you should put your .znc directory.
Run once like:
docker run --rm --user=UID-with-write-to-znc-state -it -v where-you-want-to-keep-znc-state:/.znc glyph/znc --makeconf
exit after generating your config, and subsequently:
docker run --user=UID-with-write-to-znc-state --restart=always --name=znc -v where-you-want-to-keep-znc-state:/.znc -p 6997:6997 glyph/znc
It EXPOSEs two ports, 6997 and 9458, in case you want different ports for plaintext and SSL; you'll need to configure things appropriately for SSL.