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/DNS-LDNS/t/key.t

@@ -0,0 +1,25 @@
+use Test::More tests => 8;
+
+use FindBin qw/$Bin/;
+
+use DNS::LDNS ':all';
+
+BEGIN { use_ok('DNS::LDNS') };
+
+my $key = new DNS::LDNS::Key(filename => "$Bin/testdata/key.private");
+ok($key, 'Created new key object from file');
+is($key->algorithm, 7, 'Algorithm is NSEC3RSASHA1');
+my $now = time;
+$key->set_inception($now);
+$key->set_expiration($now + 10000);
+is($key->inception, $now, 'Inception time');
+is($key->expiration, $now + 10000, 'Expiration time');
+like($key->to_rr->to_string, qr|3600\s+IN\s+DNSKEY\s+256\s+3\s+7\s+AwEAAfg/ghOkk|, 'Got rr representation of key');
+
+my $klist = new DNS::LDNS::KeyList;
+$klist->push($key);
+is($klist->count, 1, 'Keylist has one key');
+is($$key, ${$klist->key(0)}, 'Key in keylist is the one we pushed');
+# FIXME: pop is buggy in ldns 1.6.12, uncomment when this starts working
+# is($klist->pop(), $$key, 'Pop key from list');
+# is($klist->count, 0, 'No keys left in list');