builder: more experimental/windows validation

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

api/server/router/build/build_routes.go

@@ -244,6 +244,10 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 		return progress.NewProgressReader(in, progressOutput, r.ContentLength, "Downloading context", buildOptions.RemoteContext)
 	}
 
+	if buildOptions.Version == types.BuilderBuildKit && !br.daemon.HasExperimental() {
+		return errdefs.InvalidParameter(errors.New("buildkit is only supported with experimental mode"))
+	}
+
 	wantAux := versions.GreaterThanOrEqualTo(version, "1.30")
 
 	imgID, err := br.backend.Build(ctx, backend.BuildConfig{

builder/builder-next/executor_windows.go

@@ -1,11 +1,21 @@
 package buildkit
 
 import (
+	"context"
 	"errors"
+	"io"
 
+	"github.com/moby/buildkit/cache"
 	"github.com/moby/buildkit/executor"
 )
 
 func newExecutor(_ string) (executor.Executor, error) {
-	return nil, errors.New("buildkit executor not implemented for windows")
+	return &winExecutor{}, nil
+}
+
+type winExecutor struct {
+}
+
+func (e *winExecutor) Exec(ctx context.Context, meta executor.Meta, rootfs cache.Mountable, mounts []executor.Mount, stdin io.ReadCloser, stdout, stderr io.WriteCloser) error {
+	return errors.New("buildkit executor not implemented for windows")
 }