I have set the "homepage" value in the package.json to "." and that will show the basic react app in build/index.js locally but the readme just shows on the public domain. The site is hudsonbasso.com and here is a link to the repo https://github.com/hbasso/wherecaniwatch/blob/master/build/index.html . It is hosted through github pages.
I tried changing the hostname to several different things and the branch name that github pages was serving up.