App Engine deployment issues


I’m putting a Sylius-based site into production in an App Engine Standard solution.
I have solved the Argon2i problem and I have set up my application following the tutorial for Symfony proposed by Google.
The application connects to the DB and seems to run but I get stuck because some commands are not executed by the Gcloud app deploy.
I can’t find my public/build folder for example in the online folders on App Engine, so I don’t have CSS/JS.
Ideally, I would like to run a yarn encore production to generate this folder as I do locally but App Engine does not allow it.
I also have problems with CSRF tokens that don’t seem to work.
Have any of you deployed to App Engine and encountered similar problems?

Thank you in advance,