Pepperminty Wiki author here. I’ve not got any experience with Docker or creating Nextcloud apps, but in the Pepperminty Wiki Gitter Channel, we observed a few things:
Although the Apache server is supposed to be serving Pepperminty Wiki on port 8080, the web browser does not appear to be able to connect - nor does curl on the command-line of the machine with the web browser on it: https://hastebin.com/ozexusepok
Through a sudo netstat -peanut | grep -i LISTEN, we established that the only thing running on IPv4 is SSH - everything else is on IPv6 (even though Nextcloud is accessible on IPv4, no server appears to be listening on IPv4 for the web browser to connect to - which is totally baffling).
As far as my own testing goes (without Nextcloud / Docker), all you need to get Pepperminty Wiki working is a PHP-enabled web server and a directory for it to sit in. Since it’s entirely built-into a single file, it should play nice with everything else on a web server - so long as said web server doesn’t mangle the query string (the bit after the ? in the url). It should also keep to the directory you put it in - unless you tell it otherwise.