Issues when trying to deploy to Heroku

[idoco]

I am trying to deploy mockus to Heroku.
The changes can be seen here - master...idoco:heroku

The build stage runs successfully but when the app starts I get the following error:

 Error: Cannot find module '/app/mockus-server/lib/index.js'
     at Function.Module._resolveFilename (internal/modules/cjs/loader.js:902:15)
     at Function.Module._load (internal/modules/cjs/loader.js:746:27)
     at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
     at internal/main/run_main_module.js:17:47 {
   code: 'MODULE_NOT_FOUND',
   requireStack: []
 }

I think I got the path wrong though I did try a couple of variations. Can you please advise?