Fix edge case in bind mount absolute path detection

`filepath.Abs` does more than just `filepath.IsAbs` - namely, `filepath.Clean`, which resolves things like `.../.` or `.../../...`, and causes even an absolute path like `/some/path/../absolute` to fail (or, in my case, `/path/to/docker/.`)

Just using `filepath.IsAbs` directly is a much cheaper check, too. :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #5068 from tianon/fix-abs-volume-check

Fix edge case in bind mount absolute path detection