Hello :)

Symfony version(s) affected: >= 4.4.0

Description
We tried to enable opcache preloading on php 7.4 but the App_KernelProdContainer.preload.php file is not being generated. For our cache we use a custom directory outside of the project-directory. This is accomplished by overriding getCacheDir in the Kernel. If we use the default implementation of getCacheDir the generated cache contains the preload file.

How to reproduce

• Override the Kernels getCacheDir-Function and return a custom cache-dir outside of the projects path.

public function getCacheDir(): string
{
	# different from kernel.project_dir
    return '/var/application/cache/'.$this->environment;
}

• run bin/console cache:clear --env=prod
• no preload-file present under /var/application/cache/prod/

Possible Solution
In the PhpDumper-Class the autoload-file for the project is determined in the getAutoloadFile-method. The autoloader-file is only returned if it's path matches the $this->targetDirRegex, which is based on the target directory for the cache. This regex would match any autoloader contained in two directories higher than the cache-dir itself. Therefore it works if your cache is stored in %PROJECT_DIR%/var/cache/prod, but not if it is stored somewhere else.
The determination of the autoloader should therefore also/only include any files found under kernel.project_dir.
If my assumptions are correct I would happily provide a pull-request and try to fix this.

Additional context

• We use /var/application/cache for our cache and /var/www/symfony for the project itself.
• We use docker's bind-mount to mount the project into the container (on dev, in production the code is contained in the image) and a standard docker volume to share the cache between different services.