Here's what I read:

I looked up prisma's latest docker hub tags at <https://hub.docker.com/r/prismagraphql/prisma/tags/> and picked 1.14-heroku (not sure what the difference is in *-heroku tags, but it sounded reasonable. Could someone from Prisma explain the difference?)
I read Heroku's docs on docker deployment and followed this section: <https://devcenter.heroku.com/articles/container-registry-and-runtime#pushing-an-existing-image>.
Here's the exact steps I ran:

Pulled the docker image w/ the tag mentioned above: docker pull prismagraphql/prisma:1.14-heroku
Looked up the image id from the image I just pulled using: docker images
Using that image id, I tagged it to my Heroku repo: docker tag b94e6d0d0939 <http://registry.heroku.com/${HEROKU_APP_NAME}/web|registry.heroku.com/${HEROKU_APP_NAME}/web>
Pushed the image to my heroku repo: docker push <http://registry.heroku.com/${HEROKU_APP_NAME}/web|registry.heroku.com/${HEROKU_APP_NAME}/web>
Finally, release this image to Heroku: heroku container:release web --app=${HEROKU_APP_NAME}
I did a bit of manual testing after the release and it seems OK, but I would caution other people from following this as I winged it :)