pkg/archive: Canonicalize stored paths

Currently pkg/archive stores nested windows files with
backslashes (e.g. `dir\`, `dir\file.txt`) and this causes
tar not being correctly extracted on Linux daemon.

This change assures we canonicalize all paths to unix
paths and add them to tar with that name independent of platform.

Fixes the following test cases for Windows CI:
- TestBuildAddFileWithWhitespace
- TestBuildCopyFileWithWhitespace
- TestBuildAddDirContentToRoot
- TestBuildAddDirContentToExistingDir
- TestBuildCopyDirContentToRoot
- TestBuildCopyDirContentToExistDir
- TestBuildDockerignore
- TestBuildEnvUsage
- TestBuildEnvUsage2

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

archive/archive_windows.go

@@ -3,9 +3,26 @@
 package archive
 
 import (
+	"fmt"
+	"strings"
+
 	"github.com/docker/docker/vendor/src/code.google.com/p/go/src/pkg/archive/tar"
 )
 
+// canonicalTarNameForPath returns platform-specific filepath
+// to canonical posix-style path for tar archival. p is relative
+// path.
+func canonicalTarNameForPath(p string) (string, error) {
+	// windows: convert windows style relative path with backslashes
+	// into forward slashes. since windows does not allow '/' or '\'
+	// in file names, it is mostly safe to replace however we must
+	// check just in case
+	if strings.Contains(p, "/") {
+		return "", fmt.Errorf("windows path contains forward slash: %s", p)
+	}
+	return strings.Replace(p, "\\", "/", -1), nil
+}
+
 func setHeaderForSpecialDevice(hdr *tar.Header, ta *tarAppender, name string, stat interface{}) (nlink uint32, inode uint64, err error) {
 	// do nothing. no notion of Rdev, Inode, Nlink in stat on Windows
 	return