build: use buildkit's gitutil package to detect remote files

BuildKit's gitutil package behaves slightly differently than moby's
urlutil, so we should rely on BuildKit's gitutil when detecting URLs to
avoid cases of accidentally producing invalid build requests that can
confuse users.

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/utils.go

@@ -8,6 +8,8 @@ import (
 	"strings"
 
 	"github.com/docker/cli/opts"
+	"github.com/docker/docker/builder/remotecontext/urlutil"
+	"github.com/moby/buildkit/util/gitutil"
 	"github.com/pkg/errors"
 )
 
@@ -20,6 +22,16 @@ const (
 	mobyHostGatewayName = "host-gateway"
 )
 
+func IsRemoteURL(c string) bool {
+	if urlutil.IsURL(c) {
+		return true
+	}
+	if _, err := gitutil.ParseGitRef(c); err == nil {
+		return true
+	}
+	return false
+}
+
 func isLocalDir(c string) bool {
 	st, err := os.Stat(c)
 	return err == nil && st.IsDir()