Fix Kerberos support in entrypoint.sh and env.example

- Update KRB5_KTNAME path to /etc/krb5.keytab for consistency
- Add KRB5_KDC_HOST variable to env.example
- Implement error handling for missing keytab file in entrypoint.sh
- Write krb5.conf configuration dynamically based on environment variables

env.example

@@ -7,5 +7,6 @@ LDAP_ADMIN_PASSWORD=changeit
 # Kerberos SASL/GSSAPI (optional)
 KERBEROS_ENABLE=0
 KRB5_REALM=EXAMPLE.ORG
+KRB5_KDC_HOST=kerberos.example.org
 KRB5_SASL_HOST=ldap.example.org
-KRB5_KTNAME=/etc/ldap/ldap.keytab
+KRB5_KTNAME=/etc/krb5.keytab