Older images (through 8.9.x) had a line at the end of the dockerfile to set ownership of the /var/www/html folder to www-data. That line is missing from the new images built with composer. Because of this, the ownership of the folders is set to root which prevents all Drupal admin toolbar functions from working. The readme should address this or the image should be corrected (php-7.4 apache-buster)