does this work better...

index.php

···
   $provider->setPds($pds);
   $token_builder = Builder::new(new JoseEncoder(), ChainedFormatter::default());
   $algorithm    = new Sha256();
-  $signing_key   = InMemory::plainText(CERT);
+  $signing_key   = InMemory::file(CERT_PATH);
   $now   = new DateTimeImmutable();
   $token = $token_builder
     ->withHeader('alg', 'ES256')