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/lua/echo.lua

@@ -0,0 +1,68 @@
+-- source the lib file with the function
+dofile("rns-lib.lua")
+
+-- echo whatever is received
+
+-- this function disfigures the packet
+function lua_packet_mangle(orig_packet)
+--	Dont do anything just mirror
+--	local rr1 = record.new_frm_str("www.miek.nl IN A 127.0.0.1")
+--	packet.push_rr(orig_packet, LDNS_SECTION_ANSWER, rr1)
+	return(packet.to_buf(orig_packet))
+end
+
+rdf_ip = rdf.new_frm_str(LDNS_RDF_TYPE_A, "127.0.0.1")
+socket = udp.server_open(rdf_ip, 5353)
+if socket == nil then
+	os.exit(EXIT_FAILURE)
+end
+
+rdf_ip_nameserver = rdf.new_frm_str(LDNS_RDF_TYPE_A, "213.154.224.39")
+
+while true do
+	-- read from the socket, this blocks...
+	wirebuf, sockaddr_from  = udp.read(socket)
+
+	-- wrap this in new functions
+	if wirebuf == nil then
+		lua_debug("nothing received")
+	else
+		-- somebody is writing
+		wirepkt = buffer.to_pkt(wirebuf)
+		packet.print(wirepkt)
+
+		wirebuf2 = packet.to_buf(wirepkt)
+
+		-- send it to /our/ nameserver
+		socket_nameserver = udp.open(rdf_ip_nameserver, 53)
+		if socket_nameserver == nil then
+			os.exit(EXIT_FAILURE)
+		end
+
+		nameserver_bytes = udp.write(socket_nameserver, wirebuf2, rdf_ip_nameserver, 53)
+		if nameserver_bytes == nil then
+			lua_debug("ns write error")
+		end
+
+		nameserver_buf, sockaddr_from_nameserver  = udp.read(socket_nameserver)
+		udp.close(socket_nameserver)
+
+		nameserver_pkt = buffer.to_pkt(nameserver_buf)
+		packet.print(nameserver_pkt)
+		
+		-- make a new buf and write that back to the client
+		nsbuf2 = lua_packet_mangle(nameserver_pkt)
+		bytes = lua_udp_write(socket, nsbuf2, sockaddr_from) --this works
+
+		if bytes == nil then
+			lua_debug("write error")
+		end
+
+		buffer.free(nsbuf2)
+		buffer.free(nameserver_buf)
+		buffer.free(wirebuf2)
+		buffer.free(wirebuf)
+		
+	end
+end
+udp.close(socket)