Cool. I’ve fixed the dockerfile-rails gem to use this going forward.
OK, so it is not your Dockerfile, and is not your Gemfile. Now lets move on to Gemfile.lock. Delete it and redeploy. The last time I asked somebody to try this it solved their problem. And did not reappear when they ran bundle install again. See: Existing Rails app errors out on fly-deploy - #11 by doctor.quinoa .