dockerfile: set default shell based on OS

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1353,7 +1353,7 @@ func withShell(img Image, args []string) []string {
 	if len(img.Config.Shell) > 0 {
 		shell = append([]string{}, img.Config.Shell...)
 	} else {
-		shell = defaultShell()
+		shell = defaultShell(img.OS)
 	}
 	return append(shell, strings.Join(args, " "))
 }

frontend/dockerfile/dockerfile2llb/defaultshell.go

@@ -0,0 +1,8 @@
+package dockerfile2llb
+
+func defaultShell(os string) []string {
+	if os == "windows" {
+		return []string{"cmd", "/S", "/C"}
+	}
+	return []string{"/bin/sh", "-c"}
+}

frontend/dockerfile/dockerfile2llb/defaultshell_unix.go

@@ -1,7 +0,0 @@
-// +build !windows
-
-package dockerfile2llb
-
-func defaultShell() []string {
-	return []string{"/bin/sh", "-c"}
-}

frontend/dockerfile/dockerfile2llb/defaultshell_windows.go

@@ -1,7 +0,0 @@
-// +build windows
-
-package dockerfile2llb
-
-func defaultShell() []string {
-	return []string{"cmd", "/S", "/C"}
-}