It was trying to omit the `-a` flag, but that wasn't enough,
because the underlying `register_new_matrix_user` command
prompts interactively if it doesn't see the `-a` flag
(it doesn't default to non-admin).

We need to answer such interactive prompts.

Doing this means that matrix-nginx-proxy now starts
occupying port 80, which necessitates that
SSL renewal happens slightly differently.