Fix #8259 - Can't reuse symlink'd bindmount

Fixes #8259

volumes.Get was not checking for symlinked paths meanwhile when adding a
new volume it was following the symlink.
So when trying to use a bind-mount that is a symlink, the volume is
added with the correct path, but when another container tries to use the
same volume it got a "Volume exists" error because volumes.Get returned
nil and as such attempted to create a new volume.