Gem not found error when deploying monorepo to App Platform

Hi,

I am using App Platform to deploy a Ruby on Rails API backend. Rails version : 7.0.2 Ruby version : 3.1.1

When I deploy my backend on App Platform both build and deploy phase are working properly.

I am switching to a monorepo where I have two folders : /frontend and /backend.

I created a new App on App platform by setting the source directory to /backend.

Then, the build phase worked well.

But when the deploy phase starts I get this error :

[2022-03-31 08:46:55] /usr/lib/ruby/2.5.0/rubygems/dependency.rb:310:in `to_specs': Could not find 'racc' (~> 1.4) among 176 total gem(s) (Gem::MissingSpecError)
[2022-03-31 08:46:55] Checked in 'GEM_PATH=/layers/heroku_ruby/gems/vendor/bundle/ruby/3.1.0:/workspace/.gem/ruby/2.5.0:/var/lib/gems/2.5.0:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.5.0:/usr/share/rubygems-integration/2.5.0:/usr/share/rubygems-integration/all', execute `gem env` for more information
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1469:in `block in activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `each'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1440:in `activate'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1472:in `block in activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `each'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1440:in `activate'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1472:in `block in activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `each'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1440:in `activate'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1472:in `block in activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `each'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1440:in `activate'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1472:in `block in activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `each'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1458:in `activate_dependencies'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1440:in `activate'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems.rb:310:in `block in activate_bin_path'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems.rb:309:in `synchronize'
[2022-03-31 08:46:55]   from /usr/lib/ruby/2.5.0/rubygems.rb:309:in `activate_bin_path'
[2022-03-31 08:46:55]   from /layers/heroku_ruby/gems/vendor/bundle/ruby/3.1.0/bin/rails:25:in `<main>'

The build command is

rails server -p $PORT -e ${RAILS_ENV:-production}

Thanks a lot for your help,

Adrien