Support windows style dockerfile paths for build cmd

Currently TestBuildRenamedDockerfile fails since passing
custom dockerfile paths like:

    docker build -f dir/file .

fails on windows because those are unix paths. Instead, on
windows accept windows style paths like:

    docker build -f dir\file .

and convert them to unix style paths using the helper we
have in `pkg/archive` so that daemon can correctly locate
the path in the context.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

archive/archive.go

@@ -175,7 +175,7 @@ type tarAppender struct {
 // canonicalTarName provides a platform-independent and consistent posix-style
 //path for files and directories to be archived regardless of the platform.
 func canonicalTarName(name string, isDir bool) (string, error) {
-	name, err := canonicalTarNameForPath(name)
+	name, err := CanonicalTarNameForPath(name)
 	if err != nil {
 		return "", err
 	}