chore(linux): document DNS control methods (#4147)

Internal (Not private, just un-interesting to most users) docs and
research to explain the DNS control methods.

I think Jamil was right, we should revert `/etc/resolv.conf` on exit in
case it's used on some minimal Debian kitten. We can keep that and the
`systemd-resolved` method around to support desktop Ubuntu. Everything
else is going to be "When someone needs it".

rust/gui-client/docs/dns-control.md

@@ -0,0 +1,45 @@
+# Conclusion
+
+- Keep the `/etc/resolv.conf` method for containers and minimal Debian installs
+- Keep `systemd-resolved` for Ubuntu desktops, likely a popular target
+- If anyone asks nicely, `nmcli` is easy, and if NetworkManager is too big a package, there may be other options.
+
+# Methods
+
+1. Rewrite `/etc/resolv.conf` in-place. Works well on disposable 'cattle' systems like Alpine containers. Has many edge cases.
+2. Cooperate with `systemd-resolved`. Works well on Ubuntu servers and desktops.
+3. Cooperate with `NetworkManager` using `nmcli`. Works well on desktop Debian.
+4. Run as a plugin for `NetworkManager`. Not implemented yet, seems complicated, but probably robust and user-friendly for NM users.
+5. Cooperate with `netplan`. Not yet researched.
+6. Cooperate with [`resolvconf`](https://wiki.debian.org/NetworkConfiguration#The_resolvconf_program) by editing `/etc/network/interfaces.d/`. Not yet researched.
+7. Cooperate with `ifupdown`, not yet researched, might not control DNS.
+
+# Distros
+
+## Containers
+
+Always edit `/etc/resolv.conf`.
+
+- Alpine 3.19, inside container - [Official policy](https://wiki.alpinelinux.org/wiki/Configure_Networking#Configuring_DNS) is to rewrite `/etc/resolv.conf`. `networkmanager`, `resolvconf`, and `ifupdown` available in package manager but untested. Systemd not supported.
+- Debian 12, inside container - Nothing installed, just edit `/etc/resolv.conf`.
+- Ubuntu 22.02, inside container - Nothing installed, just edit `/etc/resolv.conf`.
+
+## Debian
+
+Allow user to opt in to editing `/etc/resolv.conf`, or ask them to install `systemd-resolved`.
+
+- Debian 12.5, CLI only - Editing `/etc/resolv.conf` [allowed if nothing is installed to coordinate it](https://wiki.debian.org/NetworkConfiguration#The_resolv.conf_configuration_file) `ifupdown` installed. Many other options available from `apt`.
+- Debian 12.5 with KDE - Cooperating with `nmcli` works well.
+
+## Ubuntu
+
+Always use `systemd-resolved`.
+
+- Ubuntu Server 20.04, CLI only - `systemd-resolved` installed. Netplan also available.
+- Ubuntu Server 22.04, CLI only - `systemd-resolved` installed. Netplan also available.
+
+## Other
+
+Not supported at this time.
+
+- Fedora Cloud 39 - Couldn't figure out how to log in yet