Failed binding port to Ruby application on Heroku - ruby-on-rails

Question

I want to host a Ruby on Rails app as a Docker container on Heroku. But it seems to have an issue with port binding when deployed.
I successfully hosted a simple Ruby app via Docker locally and on Heroku then I moved to the actual app. The app works fine on localhost but it gets the port binding issue:
Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2019-05-06T18:23:13.608790+00:00 app[web.1]: Digest::Digest is deprecated; use Digest
2019-05-06T18:23:15.826137+00:00 app[web.1]: Puma 2.7.1 starting...
2019-05-06T18:23:15.826185+00:00 app[web.1]: * Min threads: 0, max threads: 16
2019-05-06T18:23:15.826188+00:00 app[web.1]: * Environment: production
2019-05-06T18:23:15.826189+00:00 app[web.1]: * Listening on tcp://0.0.0.0:3000
2019-05-06T18:24:03.338792+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2019-05-06T18:24:03.338920+00:00 heroku[web.1]: Stopping process with SIGKILL
2019-05-06T18:24:03.468261+00:00 heroku[web.1]: State changed from starting to crashed
2019-05-06T18:24:03.451392+00:00 heroku[web.1]: Process exited with status 137
2019-05-06T18:24:05.407120+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=floating-spire-20546.herokuapp.com request_id=9362ea3c-bcb6-476a-a5d4-76c82cd443a3 fwd="5.151.93.202" dyno= connect= service= status=503 bytes= protocol=https
I tried configuring $PORT variable on Heroku to 3000. I also tried running the app with rails server -b 0.0.0.0. I tried without any port too.
I am thinking it may be due to the size of the app and multiple gems to be loaded which may exceed 60 seconds.
I expect the app to bind to the port to be accessed. However, it crashed.

Answer 1

I tried configuring $PORT variable on Heroku to 3000
That's not how the PORT environment variable works. You don't get to set it to the value you want to use; Heroku sets it and you must bind to the port it gives you.
For example, if you're using Puma you might include the -p option in your Procfile:
web: bundle exec puma -p ${PORT:-3000}
Here we use the value given by $PORT, falling back to 3000 in case it isn't set (e.g. on your development machine).

Answer 2

In my case, I had to use a different startup script.
I used CMD ["bundle", "exec", "puma" ,"-C" ,"config/puma.rb"]
instead of rails server -b 0.0.0.0