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_test.go

@@ -0,0 +1,48 @@
+// +build windows
+
+package archive
+
+import (
+	"testing"
+)
+
+func TestCanonicalTarNameForPath(t *testing.T) {
+	cases := []struct {
+		in, expected string
+		shouldFail   bool
+	}{
+		{"foo", "foo", false},
+		{"foo/bar", "___", true}, // unix-styled windows path must fail
+		{`foo\bar`, "foo/bar", false},
+		{`foo\bar`, "foo/bar/", false},
+	}
+	for _, v := range cases {
+		if out, err := canonicalTarNameForPath(v.in); err != nil && !v.shouldFail {
+			t.Fatalf("cannot get canonical name for path: %s: %v", v.in, err)
+		} else if v.shouldFail && err == nil {
+			t.Fatalf("canonical path call should have pailed with error. in=%s out=%s", v.in, out)
+		} else if !v.shouldFail && out != v.expected {
+			t.Fatalf("wrong canonical tar name. expected:%s got:%s", v.expected, out)
+		}
+	}
+}
+
+func TestCanonicalTarName(t *testing.T) {
+	cases := []struct {
+		in       string
+		isDir    bool
+		expected string
+	}{
+		{"foo", false, "foo"},
+		{"foo", true, "foo/"},
+		{`foo\bar`, false, "foo/bar"},
+		{`foo\bar`, true, "foo/bar/"},
+	}
+	for _, v := range cases {
+		if out, err := canonicalTarName(v.in, v.isDir); err != nil {
+			t.Fatalf("cannot get canonical name for path: %s: %v", v.in, err)
+		} else if out != v.expected {
+			t.Fatalf("wrong canonical tar name. expected:%s got:%s", v.expected, out)
+		}
+	}
+}