There's no default value set in the code, only when a docker container gets started, the invite code gets set to convos. Ideally, if no invite code is set, a random one should be set. I had situations in mind where people fire up a convos container, forget about it and a simple /whois almost-any-user-in-the-convos-irc-channel gives me a convos host to freely sign up and do bad stuff with. :)
Just setting it to 'convos' is obviously also not really a protection against that. Should I remove that docker default for now?