What steps will reproduce the problem?
1. Add your repo as per https://prosody.im/download/package_repository#debianubuntu
2. Install e.g. prosody-0.10
3. ls -l /etc/prosody/prosody.cfg.lua
Please provide any additional information below.
Debian's own downstream package installs the file as root:prosody 640, but this prevents unprivileged accounts from reading it. I guess you want it readable for all, but it shouldn't require executable bit set, because it's a config file. So instead of 755 I suggest 644, which also works (checked here) for unprivileged accounts trying to run prosodyctl check, for example.
We can look at this when we get to 0.10 packaging