bake: raise maximum size limit and fix size check

Similar to docker#2716.

Use the file size rather than the proto size, raise the allowed limit to
the same value for consistency, and improve the error message to include
the limit in human units.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

bake/remote.go

@@ -11,15 +11,17 @@ import (
 	controllerapi "github.com/docker/buildx/controller/pb"
 	"github.com/docker/buildx/driver"
 	"github.com/docker/buildx/util/progress"
+	"github.com/docker/go-units"
 	"github.com/moby/buildkit/client"
 	"github.com/moby/buildkit/client/llb"
 	"github.com/moby/buildkit/frontend/dockerui"
 	gwclient "github.com/moby/buildkit/frontend/gateway/client"
 	"github.com/moby/buildkit/session"
 	"github.com/pkg/errors"
-	"google.golang.org/protobuf/proto"
 )
 
+const maxBakeDefinitionSize = 2 * 1024 * 1024 // 2 MB
+
 type Input struct {
 	State *llb.State
 	URL   string
@@ -178,9 +180,9 @@ func filesFromURLRef(ctx context.Context, c gwclient.Client, ref gwclient.Refere
 	name := inp.URL
 	inp.URL = ""
 
-	if len(dt) > proto.Size(stat) {
-		if proto.Size(stat) > 1024*512 {
-			return nil, errors.Errorf("non-archive definition URL bigger than maximum allowed size")
+	if int64(len(dt)) > stat.Size {
+		if stat.Size > maxBakeDefinitionSize {
+			return nil, errors.Errorf("non-archive definition URL bigger than maximum allowed size (%s)", units.HumanSize(maxBakeDefinitionSize))
 		}
 
 		dt, err = ref.ReadFile(ctx, gwclient.ReadRequest{