keep an eye on AIO-Talk on the AIO GitHub ā canāt say how long it will takeā¦ usually when its ready for release 
yes you can install the app ā youāll have to configure the service GitHub - nextcloud/notify_push: Update notifications for nextcloud clients Ā· GitHub or run AIO which has client push included
that message is informative, its not an error or a warning. dunno if its a problem for you, do you need push notifications?
it is useful though ā personally I use Talk quite a bit and the notifications help keep track of incoming messages, especially if youāre using the Talk app on your cellphone together with Nextcloud services, which actually pushes messages from Nextcloud to your cell.
its also useful for the Desktop client and helps keep track of file changes (HPB:Files), but Iāve never needed a Desktop client or a sync client at all.
your setup is not clear, but obviously you know your way around dockerā¦ so you should be able to include it in your stack ā check out the AIO stack and add notify_push to your stack.
for the Nextcloud snap its a piece of cake to set up client push with a systemd notify_push service: