Stumbled across this thread. As 0xTim says in the beginning of the thread, I as a new user got stuck on this for a considerable amount of time.
Because I develop on RHEL and the binaries are not available for my OS, I am forced to use an Ubuntu container. Following the documentation gets me stuck at being unable to access the hello world page from the container host.
Personally I do not see the problem with defaulting to 0.0.0.0 because I do not open the assigned port on the firewall anyway and the app would be served to outside the network by a lighttpd web server reverse proxy.
A workstation and development environment on linux comes with at least a firewall. Access control is also tight with SELinux.
If 127.0.0.1 is kept default for the next versions it should be mentioned in the documentation at least, in order to reduce frustration to new users.