Add a more granular container detection (#2501)

Add a more granular container detection

Replace is_in_container by  get_container_info
Currently detect Official container (once they update a version with a tagfile) and LinuxServer container
All other container are marked as Unknown

Report two field :
- container_type ( none | Official | LinuxServer | Unknown
- container_version ( build Version number like 16.2.0a, only reported if Official container is detected )

admin/themes/default/template/maintenance_env.tpl

@@ -17,6 +17,9 @@ const unit_MB = "{"%s MB"|@translate}"
     <li>{'Installed on %s, %s'|translate:$INSTALLED_ON:$INSTALLED_SINCE}</li>
 {/if}
     <li>{'Operating system'|@translate}: {$OS}</li>
+    {if {$CONTAINER_INFO} neq 'none'}
+      <li>{'Container info'|translate}: {$CONTAINER_INFO}</li>
+    {/if}
     <li>PHP: {$PHP_VERSION} (<a href="{$U_PHPINFO}" class="externalLink">{'Show info'|@translate}</a>)  [{$PHP_DATATIME}]</li>
     <li>{$DB_ENGINE}: {$DB_VERSION} [{$DB_DATATIME}]</li>
     {if isset($GRAPHICS_LIBRARY)}