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/examples/ldns-key2ds.1

@@ -0,0 +1,52 @@
+.TH ldns-key2ds 1 "30 May 2005"
+.SH NAME
+ldns-key2ds \- transform a DNSKEY RR to a DS RR
+.SH SYNOPSIS
+.B ldns-key2ds 
+.IR file
+
+.SH DESCRIPTION
+\fBldns-key2ds\fR is used to transform a public DNSKEY RR to a DS RR.
+When run it will read \fIfile\fR with a DNSKEY RR in it and
+it will create a .ds file with the DS RR in it.
+
+It prints out the basename for this file (K<name>+<alg>+<id>).
+
+By default it takes a pick of algorithm similar to the key algorithm,
+SHA1 for RSASHA1, and so on.
+
+.SH OPTIONS
+.TP
+\fB-f\fR
+Ignore SEP flag (i.e. make DS records for any key)
+
+.TP
+\fB-n\fR
+Write the result DS Resource Record to stdout instead of a file
+
+.TP
+\fB-1\fR
+Use SHA1 as the hash function.
+
+.TP
+\fB-2\fR
+Use SHA256 as the hash function
+
+.TP
+\fB-g\fR
+Use GOST as the hash function
+
+.TP
+\fB-4\fR
+Use SHA384 as the hash function
+
+.SH AUTHOR
+Written by the ldns team as an example for ldns usage.
+
+.SH REPORTING BUGS
+Report bugs to <dns-team@nlnetlabs.nl>.
+
+.SH COPYRIGHT
+Copyright (C) 2005 NLnet Labs. This is free software. There is NO
+warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
+PURPOSE.