04cf68b45e make nginx responsible for downloading (and caching)
files from S3. As noted in that commit, nginx implements its own
non-blocking DNS resolver, since the base syscall is blocking, so
requires an explicit nameserver configuration. That commit used
127.0.0.53, which is provided by systemd-resolved, as the resolver.
However, that service may not always be enabled and running, and may
in fact not even be installed (e.g. on Docker). Switch to parsing
`/etc/resolv.conf` and using the first-provided nameserver. In many
deployments, this will still be `127.0.0.53`, but for others it will
provide a working DNS server which is external to the host.
In the event that a server is misconfigured and has no resolvers in
`/etc/resolv.conf`, it will error out:
```console
Error: Evaluation Error: Error while evaluating a Function Call, No nameservers found in /etc/resolv.conf! Configure one by setting application_server.nameserver in /etc/zulip/zulip.conf (file: /home/zulip/deployments/current/puppet/zulip/manifests/app_frontend_base.pp, line: 76, column: 70) on node example.zulipdev.org
```
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