Yes, that was exactly what i wanted to do and how i setupped the server/admin.js following your guide (thanks, first time on strapi/docker). Too bad Heroku isn’t my personal localhost 
but i’ve got similar problems with strapi also back in 2018 as CTO of the last official spinoff of the first official (and only they were saying) microsoft research center in EU. I googled a bit, and saw all that similar comments of these last days, as in your tutorial and issue link… so i lurked out asking about actual situation on official channels. I’ll try to reconfig with a plain strapi img and heroku app from scratch, maybe something from past dockerizations and integrations went and remained wrong. Not first time an img got corrupted. 
I’ll keep you informed. Thanks again!