alertmanager down with "create memberlist: Failed to get final advertise address: Failed to parse advertise address \"<nil>\""

Summary

When starting GitLab in an LXD container with netmask set to 255.255.255.255, alertmanager fails to start.

Steps to reproduce

$ sudo apt-get install -t xenial-backports lxd lxd-client
$ sudo lxc launch ubuntu:16.04 c1
$ sudo lxc exec c1 /bin/bash
$ sudo cat >/etc/network/interfaces.d/eth0.cfg <<EOF
auto eth0
iface eth0 inet static
  address X.Y.W.Z
  netmask 255.255.255.255
  gateway X.Y.W.1
EOF
$ exit
$ sudo lxc restart c1
$ sudo lxc exec c1 /bin/bash
$ # Install GitLab CE according to https://about.gitlab.com/installation/#ubuntu
$ sudo gitlab-ctl status
down: alertmanager: 0s, normally up, want up; run: log: (pid 209) 2428s
run: crond: (pid 192) 2428s; run: log: (pid 190) 2428s
run: gitaly: (pid 206) 2428s; run: log: (pid 203) 2428s
run: gitlab-monitor: (pid 222) 2428s; run: log: (pid 220) 2428s
run: gitlab-workhorse: (pid 201) 2428s; run: log: (pid 196) 2428s
run: logrotate: (pid 212) 2428s; run: log: (pid 211) 2428s
run: nginx: (pid 205) 2428s; run: log: (pid 200) 2428s
run: node-exporter: (pid 226) 2428s; run: log: (pid 215) 2428s
run: postgres-exporter: (pid 225) 2428s; run: log: (pid 210) 2428s
run: postgresql: (pid 224) 2428s; run: log: (pid 202) 2428s
run: prometheus: (pid 9300) 518s; run: log: (pid 188) 2428s
run: redis: (pid 197) 2428s; run: log: (pid 193) 2428s
run: redis-exporter: (pid 208) 2428s; run: log: (pid 207) 2428s
run: sidekiq: (pid 9061) 530s; run: log: (pid 198) 2428s
run: unicorn: (pid 9553) 511s; run: log: (pid 194) 2428s

What is the current bug behavior?

alertmanager fails to start and infinitely tries to restart.

What is the expected correct behavior?

alertmanager should start and stay in up state

Relevant logs and/or screenshots

2018-07-27_11:19:17.43890 level=info ts=2018-07-27T11:19:17.438801901Z caller=main.go:174 msg="Starting Alertmanager" version="(version=0.15.0, branch=master, revision=8a7fce686f9208a1c146a3ff6e937f4ba4a2678b)"
2018-07-27_11:19:17.43896 level=info ts=2018-07-27T11:19:17.43886827Z caller=main.go:175 build_context="(go=go1.10.3, user=GitLab-Omnibus, date=2018-07-26T01:08:31+00:00)"
2018-07-27_11:19:17.44422 level=info ts=2018-07-27T11:19:17.444135113Z caller=cluster.go:155 component=cluster msg="setting advertise address explicitly" addr=<nil> port=9094
2018-07-27_11:19:17.44463 level=error ts=2018-07-27T11:19:17.444578438Z caller=main.go:201 msg="Unable to initialize gossip mesh" err="create memberlist: Failed to get final advertise address: Failed to parse advertise address \"<nil>\""

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 16.04
Current User:   git
Using RVM:      no
Ruby Version:   2.4.4p296
Gem Version:    2.7.6
Bundler Version:1.16.2
Rake Version:   12.3.1
Redis Version:  3.2.11
Git Version:    2.17.1
Sidekiq Version:5.1.3
Go Version:     unknown
GitLab information
Version:        11.1.2
Revision:       35936b0
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://git.example.com
HTTP Clone URL: https://git.example.com/some-group/some-project.git
SSH Clone URL:  git@git.example.com:some-group/some-project.git
Using LDAP:     no
Using Omniauth: no
GitLab Shell
Version:        7.1.4
Repository storage paths:
‐ default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks
Git:            /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab Shell ...
GitLab Shell version >= 7.1.4 ? ... OK (7.1.4)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
...
90/660 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.4)
Git version >= 2.9.5 ? ... yes (2.17.1)
Git user has default SSH configuration? ... yes
Active users: ... 7
Checking GitLab ... Finished