credentialprovider: Sets homeDirPath to os.UserHomeDir()

Currently, the credential provider will look in the path set in
the $HOME env variable, but that environment does not exist on
Windows, but $HOMEPATH does. Because of this, if credentials are
set in ~/.docker on Windows, they will not be used by kubelet
when pulling images.

The function os.UserHomeDir can solve this problem [1].

[1] https://golang.org/pkg/os/#UserHomeDir

pkg/credentialprovider/config.go

@@ -60,7 +60,7 @@ var (
 	preferredPathLock sync.Mutex
 	preferredPath     = ""
 	workingDirPath    = ""
-	homeDirPath       = os.Getenv("HOME")
+	homeDirPath, _    = os.UserHomeDir()
 	rootDirPath       = "/"
 	homeJsonDirPath   = filepath.Join(homeDirPath, ".docker")
 	rootJsonDirPath   = filepath.Join(rootDirPath, ".docker")