Document the undocumented HOSTNAME variable

README.md

@@ -310,6 +310,7 @@ Other environment variables:
 
 - **LDAP_REMOVE_CONFIG_AFTER_SETUP**: delete config folder after setup. Defaults to `true`
 - **LDAP_SSL_HELPER_PREFIX**: ssl-helper environment variables prefix. Defaults to `ldap`, ssl-helper first search config from LDAP_SSL_HELPER_* variables, before SSL_HELPER_* variables.
+- **HOSTNAME**: set the hostname of the running openldap server. Defaults to whatever docker creates.
 
 
 ### Set your own environment variables