If you don't know why it's needed, I recommend following the instructions. :)

Having an empty wgScriptPath is not recommended.
https://www.mediawiki.org/wiki/Manual:Wiki_in_site_root_directory

Other settings may be needed e.g. for VisualEditor and caching to work correctly.

On Ubuntu, just installing the Debian packages should suffice. If you don't have strong opinions about nginx you can just use the Apache defaults.
https://www.mediawiki.org/wiki/Debian_packages

given that you're using Docker, I suspect some of the configuration that's listed here must go in the configuration of the web server that is running inside the mediawiki container, not the one that's outside.

so it's probably easiest to find some instructions specifically tailored to MediaWiki deployments with the Docker image you are using

Ah yes definitely, must follow the instructions for your installation method.

thanks both!

Did a bit of digging on your suggestions... Indeed, the mediawiki:1.42 docker image comes with apache and shortlinks enabled

https://github.com/wikimedia/mediawiki-docker/blob/b3b4ecdcb503a180b1f5052b04963a0fb93f6611/1.42/apache/Dockerfile#L55

(I understand now that a docker container must necessarily have a webserver installed!)

So I created https://www.mediawiki.org/wiki/Manual:Short_URL/Docker and linked it the general page for short URLs

Would be happy to receive friendly criticism before the unfriendly criticism 🙃

I think that's helpful! Maybe the inclusion in https://www.mediawiki.org/wiki/Manual:Short_URL#Guides can be tweaked somewhat - on the surface it seems to imply that Docker is a web server, so maybe that can be improved…