How to override with conf.d? about inspircd-docker

[genius3000]

I don't think overriding works here, at least I know it doesn't with a non-Docker install of InspIRCd, assuming Docker install would be the same. That'd explain receiving an error trying to use the name 'main', guessing the error is for the duplicate name.
In fact, the Docker notes clarify under Generic configuration includes:

You have to take care about possible conflicts with the existing configuration. If you want a full custom configuration, copy or mount it to /inspircd/conf/ instead of /inspircd/conf.d/.

As for the password not seeming to take effect, I'm guessing all your connections are ending up in the default 'main' connect class (allow all, port 6667). Connect classes are checked from top to bottom, likely the include for the 'conf.d' folder happens after that 'main' class is defined.