Create /etc/resolv.conf in initramfs for tailscale client

On Debian systems, unless configured by something else, there isn't an
/etc/resolv.conf in the initramfs, which hinders DNS resolution.
Tailscale has its fallback DNS functionality, but that has problems [1]
in the corner-case situation of a Debian initramfs environment:

- no /etc/resolv.conf means tailscale (or golang?) attempts to use
  127.0.0.1 as a DNS resolver
- the loopback interface (lo) isn't brought up in the initramfs. linux
  sends the DNS traffic off-device (destined for 127.0.0.1)

Even with the Tailscale fix for that issue, it's a fallback and there's
a noticeable delay, so do the correct thing here and create
/etc/resolv.conf.

[1] https://github.com/tailscale/tailscale/issues/6110

config/config

@@ -43,6 +43,18 @@ TAILSCALE_AUTHKEY=
 #
 #TAILSCALE_LOGOUT=
 
+#
+# Space-delimited list of DNS server(s) to put in /etc/resolv.conf in the
+# initramfs if /etc/resolv.conf doesn't exist and if no DNS servers were
+# returned from DHCP.
+#
+# Tailscale will eventually fall back to bootstrapping DNS itself, but there's
+# no reason to rely on that (adds a slight delay).  Tailscale's fallback is
+# also problematic in some corner cases in some versions (see
+# https://github.com/tailscale/tailscale/issues/6110).
+#
+#FALLBACK_DNS_SERVERS=
+
 #
 # Bring down interfaces matching this pattern before passing out of the
 # initramfs.  (Same behavior as dropbear-initramfs)