Set DNSStubListenerExtra

**What I'd like:**

I would like to set systemd-resolve [DNSStubListenerExtra](https://www.freedesktop.org/software/systemd/man/latest/resolved.conf.html#DNSStubListenerExtra=).

Specifically to the Docker gateway IP, to so that orchestrated containers have cached DNS responses from systemd-resolve.

**Any alternatives you've considered:**

I use a host container to modify /etc/systemd/resolved.conf. That is unnecessarily fragile.

Another workaround is to configure, package, and deploy a separate DNS forwarder as a daemon or host container, such as dnsmasq or CoreDNS, which run in host networking mode, and forward queries to systemd-resolve. That is unnecessarily complex.