The dokku-push action requires an SSH key with push access to the Dokku instance. Here's how to do that.
Replace APPNAME
with the name of the app (e.g. this is a good idea to use the same name used on Dokku's).
We want each repo to have its own SSH key, so it's easier to rotate/invalidate them if required, without affecting all the repos. Let's generate a new key on your computer (see GitHub help):