nixos/doc: mention "No such file or directory" error on Nextcloud (#342353)

    To work around that, please make sure that all directories in question

    are owned by `nextcloud:nextcloud`.

  - **`Failed to open stream: No such file or directory` after deploys**

    Symptoms are errors like this after a deployment that disappear after

    a few minutes:

    ```

    Warning: file_get_contents(/run/secrets/nextcloud_db_password): Failed to open stream: No such file or directory in /nix/store/lqw657xbh6h67ccv9cgv104qhcs1i2vw-nextcloud-config.php on line 11

    Warning: http_response_code(): Cannot set response code - headers already sent (output started at /nix/store/lqw657xbh6h67ccv9cgv104qhcs1i2vw-nextcloud-config.php:11) in /nix/store/ikxpaq7kjdhpr4w7cgl1n28kc2gvlhg6-nextcloud-29.0.7/lib/base.php on line 639

    Cannot decode /run/secrets/nextcloud_secrets, because: Syntax error

    ```

    This can happen if [](#opt-services.nextcloud.secretFile) or

    [](#opt-services.nextcloud.config.dbpassFile) are managed by

    [sops-nix](https://github.com/Mic92/sops-nix/).

    Here, `/run/secrets/nextcloud_secrets` is a symlink to

    `/run/secrets.d/N/nextcloud_secrets`. The `N` will be incremented

    when the sops-nix activation script runs, i.e.

    `/run/secrets.d/N` doesn't exist anymore after a deploy,

    only `/run/secrets.d/N+1`.

    PHP maintains a [cache for `realpath`](https://www.php.net/manual/en/ini.core.php#ini.realpath-cache-size)

    that still resolves to the old path which is causing

    the `No such file or directory` error. Interestingly,

    the cache isn't used for `file_exists` which is why this warning

    comes instead of the error from `nix_read_secret` in

    `override.config.php`.

    One option to work around this is to turn off the cache by setting

    the cache size to zero:

    ```nix

    services.nextcloud.phpOptions."realpath_cache_size" = "0";

    ```

## Using an alternative webserver as reverse-proxy (e.g. `httpd`) {#module-services-nextcloud-httpd}

By default, `nginx` is used as reverse-proxy for `nextcloud`.