Hey @openHBP / @Nonsequitur !
I was wondering if you were able to use Enketo (for form previews) when hosted ODK Central (using docker, not the development environment) locally? I tried the same and I am stuck @ form server could not be found (also here: Could not connect with Form Server Preview). Also, I am not sure if I am correct but it seems like the http (non SSL) access to Central no more works with the latest release and is blocked via server block in nginx?! Then how come did the localhost thing work with "upstream"?! 
Please let me know..