Leave mount type as nil for Windows

		-Added a check to exclude Windows OS when adding bind mount types to container image bundle.
    -Bind types apply to Linux OS; hcsshim accepts empty bind types for Windows container creation.

Signed-off-by: Billy Owire <billyowire95@gmail.com>

frontend/dockerfile/docs/reference.md

@@ -842,7 +842,7 @@ environment variable with the same name.
 # syntax=docker/dockerfile:1
 FROM alpine
 RUN --mount=type=secret,id=API_KEY,env=API_KEY \
-    some-command --token-from-env API_KEY
+    some-command --token-from-env $API_KEY
 ```
 
 Assuming that the `API_KEY` environment variable is set in the build

frontend/gateway/gateway.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -400,11 +401,14 @@ type bindMount struct {
 }
 
 func (b *bindMount) Mount() ([]mount.Mount, func() error, error) {
-	return []mount.Mount{{
-		Type:    "bind",
-		Source:  b.dir,
-		Options: []string{"bind", "ro", "nosuid", "nodev", "noexec"},
-	}}, func() error { return nil }, nil
+	if runtime.GOOS != "windows" {
+		return []mount.Mount{{
+			Type:    "bind",
+			Source:  b.dir,
+			Options: []string{"bind", "ro", "nosuid", "nodev", "noexec"},
+		}}, func() error { return nil }, nil
+	}
+	return nil, func() error { return nil }, nil
 }
 func (b *bindMount) IdentityMapping() *idtools.IdentityMapping {
 	return nil