All you need to do simply enable port 80, you can use this command:
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dokku ports:add my-app https:443:8080
dokku ports:add my-app http:80:8080
If you debug with dokku ports:report my-app
it should showing result like this:
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=====> my-app ports information
Ports map: http:80:8080 https:443:8080
Ports map detected: http:80:5000 https:443:5000
The port 5000 seems generated automatically and cannot be deleted. That's it!