Where do you store user uploaded content within a Symfony 4 application?

I have a section within my site where the user can upload their own profile pictures which is stored in the output directory and tracked in the database like so:

    $form = $this->createForm(ProfileUpdateForm::class);
    $form->handleRequest($request);
    if ($form->isSubmitted() && $form->isValid())
    {
        $user = $this->getUser();
        $firstname = $form->get('firstname')->getData();
        $lastname = $form->get('lastname')->getData();
        $picture = $form->get('profilepicture')->getData();

        if($picture == null)
        {
            $user
            ->setFirstName($firstname)
            ->setLastName($lastname);
        }
        else
        {
            $originalFilename = pathinfo($picture->getClientOriginalName(), PATHINFO_FILENAME);
            // this is needed to safely include the file name as part of the URL
            $safeFilename = strtolower(str_replace('', '', $originalFilename));
            $newFilename = $safeFilename.'-'.uniqid().'.'.$picture->guessExtension();

            try {
                $picture->move(
                    'build/images/user_profiles/',
                    $newFilename
                );
            } catch (FileException $e) {
                $this->addFlash("error", "Something happened with the file upload, try again.");
                return $this->redirect($request->getUri());
            }

            // updates the 'picture' property to store the image file name
            // instead of its contents
            $user
            ->setProfilePicture($newFilename)
            ->setFirstName($firstname)
            ->setLastName($lastname);
        }

        $entityManager = $this->getDoctrine()->getManager();
        $entityManager->persist($user);
        $entityManager->flush();

        $this->addFlash("success", "Your profile was updated!");
        return $this->redirectToRoute('account');
    }

    return $this->render('account/profile.html.twig', [
        'profileform' => $form->createView()
    ]);

That issue I've found is that every time I compile my (local) project, the image is then deleted (because the public/build directory gets built by deleting and creating again).

If I'm not mistaken, isn't that how deployments work too? And if so, is that the right way to upload an image? What's the right way of going about this?