Symfony Nelmio : the doc page is blank in Google App Engine

Question

I uploaded my Symfony API on Google Cloud's App Engine and the API part works well. However, the documentation (handled with Nelmio) appears like this :

The view of the doc in production

The resources appear not reachable and being all new to App Engine, I'm not sure how to tackle the problem.

Here's my app.yaml :

runtime: php
env: flex

runtime_config:
  operating_system: "ubuntu22"
  document_root: public

env_variables:
  APP_ENV: 'prod'