When restoring volumes at startup, the local volume driver's constructor
([daemon/volume/local.New]) iterates over all directories found inside the
volume storage path (`/var/lib/docker/volumes`). It does not (currently) check
for presence of other indicators that the directory is an actual volume, such
as the `_data` directory being present, or a `opts.json`.

In situations where `/var/lib/docker/volumes` contains directories that were
not created by docker, this can result in errors when calculating the size
of volumes (`docker system df`);

Before (re)starting the daemon, create some directories;

    mkdir /var/lib/docker/volumes/notavolume
    echo "some file" > /var/lib/docker/volumes/notavolume/some-file
    mkdir /var/lib/docker/volumes/notavolume2

Then, start the daemon, and run `docker system df`. The daemon logs will
now contain warnings about the path not being found:

    WARN[2026-02-09T11:27:31.940713593Z] Failed to determine size of volume            error="lstat /var/lib/docker/volumes/notavolume/_data: no such file or directory" volume=notavolume
    WARN[2026-02-09T11:27:31.940765468Z] Failed to determine size of volume            error="lstat /var/lib/docker/volumes/notavolume2/_data: no such file or directory" volume=notavolume2

The [calcSize] function used to calculate the size of a volume already ignores
files _within_ the volume's storage that are not / no longer found, but does
not ignore errors if the base directory (`_data`) itself doesn't exist.

This patch:

- makes the logs ignore "not found" errors
- adds a TODO to look into ignoring these directories during daemon startup
  (when instantiating the driver and restoring volumes).

[daemon/volume/local.New]: https://github.com/moby/moby/blob/6c5233e1098dc689b2e665087780695ac8864e95/daemon/volume/local/local.go#L51-L85
[calcSize]: https://github.com/moby/moby/blob/daeaac0d3cf147cc7450a3ab9a869327c71bad45/daemon/internal/directory/directory_unix.go#L13-L24

Signed-off-by: Sebastiaan van Stijn <[email protected]>