Puma server inside heroku container fails to connect to port

Question

I have a docker container: (called Dockerfile.web, relevant parts)

ARG PORT=3000
ENV PORT=$PORT

EXPOSE $PORT

ENTRYPOINT ["docker-entrypoint.sh"]
CMD ["rails", "server"]

Puma: config/puma.rb

before_fork do
  ActiveRecord::Base.connection_pool.disconnect! if defined? ActiveRecord
end

on_worker_boot do
  ActiveRecord::Base.establish_connection if defined? ActiveRecord
end

preload_app!

So when I run heroku container:push web followed by heroku container:release web, the app deploys, but the puma server doesn't come up b/c the port cannot be connected to:

Booting Puma
Rails 5.2.2 application starting in development 
Run `rails server -h` for more startup options
 Puma starting in cluster mode...
 * Version 3.12.0 (ruby 2.6.0-p0), codename: Llamas in Pajamas
 * Min threads: 5, max threads: 5
 * Environment: development
 * Process workers: 1
 * Preloading application
 * Listening on tcp://localhost:41626
 Use Ctrl-C to stop
 - Worker 0 (pid: 28) booted, phase: 0
Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
Stopping process with SIGKILL

I'm following the official guide but I haven't added the sections:

rackup      DefaultRackup
port        ENV['PORT']     || 3000
environment ENV['RACK_ENV'] || 'development'

Where could my error be? Any ideas?

Answer 1

I'm not sure why you left this part out:

port        ENV['PORT']     || 3000

You can't pick your own port. Use the $PORT that Heroku gives you.