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.
10 jobs for !93 with topic/default/gitlab-13.12 in 21 minutes and 21 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
passed #207656
flake8

00:00:10

 
  Test Heptapod
passed #207663
docker-fully-native-inside

00:17:51

passed #207664
docker-fully-native-remote-0

00:15:58

passed #207665
docker-fully-native-remote-1

00:19:46

passed #207657
docker-inside

00:19:33

passed #207660
docker-native-inside

00:16:20

passed #207661
docker-native-remote-0

00:19:29

passed #207662
docker-native-remote-1

00:21:10

passed #207658
docker-remote-0

00:16:57

passed #207659
docker-remote-1

00:21:06