feat(config): expose allow_subadmins via OWNCLOUD_ALLOW_SUBADMINS (#494)

* feat(config): expose allow_subadmins via OWNCLOUD_ALLOW_SUBADMINS

ownCloud core (oC 11) disables the subadmin/group-admin feature by
default via the new allow_subadmins config setting. Expose it through
the OWNCLOUD_ALLOW_SUBADMINS environment variable so operators can opt
the feature back in without bind-mounting a custom config.

This is an ownCloud 11 only setting, so the change applies to v24.04
only; v20.04 and v22.04 are intentionally left unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>

* docs: clarify OWNCLOUD_ALLOW_SUBADMINS usage and deep-link docs

Address review feedback on PR #494: state that only the literal value
`true` enables the feature and that leaving it unset keeps it disabled
(ownCloud core default), and deep-link to the specific config sample
parameter section like the neighbouring entries.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>

---------

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DeepDiver1975

ENVIRONMENT.md

@@ -18,6 +18,8 @@
   ownCloud admin password.
 - `OWNCLOUD_ADMIN_USERNAME=admin` \
   ownCloud admin username.
+- `OWNCLOUD_ALLOW_SUBADMINS=` \
+  Enable or disable the subadmin (group admin) feature. Set to `true` to enable; unset leaves it disabled (ownCloud core default) (see [documentation](https://doc.owncloud.com/server/latest/admin_manual/configuration/server/config_sample_php_parameters.html#allow-or-disallow-the-group-administrator-subadmin-feature)).
 - `OWNCLOUD_ALLOW_USER_TO_CHANGE_DISPLAY_NAME=` \
   Allow or disallow users to change their display names (see [documentation](https://doc.owncloud.com/server/latest/admin_manual/configuration/server/config_sample_php_parameters.html#allow-or-disallow-users-to-change-their-display-names)).
 - `OWNCLOUD_ALLOW_USER_TO_CHANGE_MAIL_ADDRESS=` \

v24.04/overlay/etc/templates/config.php

@@ -82,6 +82,10 @@ function getConfigFromEnv() {
     $config['allow_user_to_change_mail_address'] = getenv('OWNCLOUD_ALLOW_USER_TO_CHANGE_MAIL_ADDRESS') === 'true';
   }
 
+  if (getenv('OWNCLOUD_ALLOW_SUBADMINS') != '') {
+    $config['allow_subadmins'] = getenv('OWNCLOUD_ALLOW_SUBADMINS') === 'true';
+  }
+
   if (getenv('OWNCLOUD_STRICT_LOGIN_ENFORCED') != '') {
     $config['strict_login_enforced'] = getenv('OWNCLOUD_STRICT_LOGIN_ENFORCED') === 'true';
   }