As is seem in the comment of `normaliseWorkdir` for windows:
```
...
// WORKDIR c:\\foo --> C:\foo
// WORKDIR \\foo --> C:\foo
...
```
However, this is not the case in the current implementation because
`filepath.FromSlash` is used and `FromSlash` does not replace multiple
separator with a single one (`file.Clean` does).
So `normaliseWorkdir` does not truly normalize workdir.
This fix changes the implementation of `normaliseWorkdir` and use
`filepath.Clean` instead of `filepath.FromSlash`.
Additional test cases have been added to the unit test.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
