fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/contrib/ldnsx/README

@@ -0,0 +1,36 @@
+LDNSX: Easy DNS (including DNSSEC) via ldns.
+
+ldns is a great library. It is a powerful tool for
+working with DNS. python-ldns it is a straight up clone of the C
+interface, however that is not a very good interface for python. Its
+documentation is incomplete and some functions don't work as
+described. And some objects don't have a full python API.
+
+ldnsx aims to fix this. It wraps around the ldns python bindings,
+working around its limitations and providing a well-documented, more
+pythonistic interface.
+
+Written by Christopher Olah <chris@xelerance.com>
+
+Examples
+========
+
+Query the default resolver for google.com's A records. Print the response
+packet.
+
+>>> import ldnsx
+>>> resolver = ldnsx.resolver()
+>>> print resolver.query("google.com","A")
+
+Print the NS records for com. from f.root-servers.net if we get a
+response, else an error message.
+
+>>> import ldnsx
+>>> pkt = ldnsx.resolver("f.root-servers.net").query("com.","NS")
+>>> if pkt:
+>>>    for rr in pkt.answer():
+>>>       print rr
+>>> else:
+>>>    print "response not received" 
+
+