fix(telnet): replace Cowrie with real busybox telnetd + rsyslog logging

Cowrie was exposing an SSH daemon on port 22 alongside the telnet service
even when COWRIE_SSH_ENABLED=false, contaminating deployments that did not
request an SSH service.

New implementation mirrors the SSH service pattern:
- busybox telnetd in foreground mode on port 23
- /bin/login for real PAM authentication (brute-force attempts logged)
- rsyslog RFC 5424 bridge piped to stdout for Docker log capture
- Configurable root password and hostname via env vars
- No Cowrie dependency

tests/test_composer.py

@@ -20,13 +20,13 @@ APT_COMPATIBLE = {
 }
 
 BUILD_SERVICES = [
-    "ssh", "http", "rdp", "smb", "ftp", "smtp", "elasticsearch",
+    "ssh", "telnet", "http", "rdp", "smb", "ftp", "smtp", "elasticsearch",
     "pop3", "imap", "mysql", "mssql", "redis", "mongodb", "postgres",
     "ldap", "vnc", "docker_api", "k8s", "sip",
     "mqtt", "llmnr", "snmp", "tftp", "conpot"
 ]
 
-UPSTREAM_SERVICES = ["telnet"]
+UPSTREAM_SERVICES: list = []
 
 
 def _make_config(services, distro="debian", base_image=None, build_base=None):