When running on the subpath, you need to set the following config

const jsreport = require('./')({ appPath: '/jsreportapp' })

When I render the template and use log statements, it executes successfully.

This means you see in the jsreport container output that report finished, but you see failure on the client side?
What if you just use some kind of postman/advanced rest client to invoke report rendering through API, this works?
I've never used openresty so not sure if there can be a problem with it.