Container: no need to specify captcha_url, auto may be enough in most cases

CONTAINER.md

@@ -153,10 +153,9 @@ Now update your ejabberd configuration file, for example:
 docker exec -it ejabberd vi conf/ejabberd.yml
 ```
 
-and add the required options:
-```
+and add this option:
+```yaml
 captcha_cmd: /opt/ejabberd-22.04/lib/captcha.sh
-captcha_url: https://localhost:5443/captcha
 ```
 
 Finally, reload the configuration file or restart the container:
@@ -164,6 +163,18 @@ Finally, reload the configuration file or restart the container:
 docker exec ejabberd ejabberdctl reload_config
 ```
 
+If the CAPTCHA image is not visible, there may be a problem generating it
+(the ejabberd log file may show some error message);
+or the image URL may not be correctly detected by ejabberd,
+in that case you can set the correct URL manually, for example:
+```yaml
+captcha_url: https://localhost:5443/captcha
+```
+
+For more details about CAPTCHA options, please check the
+[CAPTCHA](https://docs.ejabberd.im/admin/configuration/basic/#captcha)
+documentation section.
+
 
 Advanced Container Configuration
 ================================