Gitlab::Database::LoadBalancing::ServiceDiscovery::UnresolvableNameserverError: could not resolve localhost

https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/612471/

IPAddr::InvalidAddressError: invalid address
  ipaddr.rb:649:in `in6_addr'
    raise InvalidAddressError, "invalid address"
  ipaddr.rb:586:in `initialize'
    @addr = in6_addr(prefix)
  gitlab/database/load_balancing/service_discovery.rb:139:in `new'
    IPAddr.new(nameserver)
  gitlab/database/load_balancing/service_discovery.rb:139:in `nameserver_ip'
    IPAddr.new(nameserver)
  gitlab/database/load_balancing/service_discovery.rb:130:in `resolver'
    nameservers: nameserver_ip(@nameserver),
...
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Gitlab::Database::LoadBalancing::ServiceDiscovery::UnresolvableNameserverError: could not resolve localhost
  gitlab/database/load_balancing/service_discovery.rb:144:in `rescue in nameserver_ip'
    raise UnresolvableNameserverError, "could not resolve #{nameserver}"
  gitlab/database/load_balancing/service_discovery.rb:138:in `nameserver_ip'
    def nameserver_ip(nameserver)
  gitlab/database/load_balancing/service_discovery.rb:130:in `resolver'
    nameservers: nameserver_ip(@nameserver),
  gitlab/database/load_balancing/service_discovery.rb:103:in `addresses_from_dns'
    resources = resolver.search(record, Net::DNS::A).answer
  gitlab/database/load_balancing/service_discovery.rb:68:in `refresh_if_necessary'
    interval, from_dns = addresses_from_dns
...
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Gitlab::Database::LoadBalancing::ServiceDiscovery::UnresolvableNameserverError: could not resolve localhost