Reverse calls: explicit IP addresses
It can happen that WSGIServer (itself based on socketserver.BaseServer) binds by default using IPv4 while the `localhost` default call address that will be stored on the Rails side for reverse calls will resolve to `::1`. Since some of the contexts for these tests only have IPv4 (Docker containters), we're binding explicitely to the all IPv4 addresses 0.0.0.0 On many Linux systems, the kernel is configured so that :: serves all IPv6 and IPv4 address (through representation of the IPv4 address space in IPv6). We'' switch to that if we turn out to need IPv6 and it works in our Docker CI contexts. For now, let's spare us another round of testing.