I just upgraded to the latest Dev version of the docker container, and ran update.php, and I'm getting the following errors:

Notice: Trying to access array offset on value of type int in element_children() (line 6659 of /var/www/html/includes/common.inc).
[above line repeated approximately a bazillion times]
Deprecated function: implode(): Passing glue string after array is deprecated. Swap the parameters in drupal_get_feeds() (line 394 of /var/www/html/includes/common.inc).