build: allow multiple exports if supported by buildkit

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

build/build.go

@@ -290,7 +290,9 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 			opt.Exports = []client.ExportEntry{{Type: "image", Attrs: map[string]string{}}}
 		}
 	default:
-		return nil, nil, errors.Errorf("multiple outputs currently unsupported")
+		if err := bopts.LLBCaps.Supports(pb.CapMultipleExporters); err != nil {
+			return nil, nil, errors.Errorf("multiple outputs currently unsupported by the current BuildKit daemon, please upgrade to version v0.13+ or use a single output")
+		}
 	}
 
 	// fill in image exporter names from tags

controller/pb/export.go

@@ -15,6 +15,7 @@ func CreateExports(entries []*ExportEntry) ([]client.ExportEntry, error) {
 	if len(entries) == 0 {
 		return nil, nil
 	}
+	var stdoutUsed bool
 	for _, entry := range entries {
 		if entry.Type == "" {
 			return nil, errors.Errorf("type is required for output")
@@ -68,10 +69,14 @@ func CreateExports(entries []*ExportEntry) ([]client.ExportEntry, error) {
 				entry.Destination = "-"
 			}
 			if entry.Destination == "-" {
+				if stdoutUsed {
+					return nil, errors.Errorf("multiple outputs configured to write to stdout")
+				}
 				if _, err := console.ConsoleFromFile(os.Stdout); err == nil {
 					return nil, errors.Errorf("dest file is required for %s exporter. refusing to write to console", out.Type)
 				}
 				out.Output = wrapWriteCloser(os.Stdout)
+				stdoutUsed = true
 			} else if entry.Destination != "" {
 				fi, err := os.Stat(entry.Destination)
 				if err != nil && !os.IsNotExist(err) {

docs/reference/buildx_build.md

@@ -424,6 +424,10 @@ $ docker buildx build -o type=docker,dest=- . > myimage.tar
 $ docker buildx build -t tonistiigi/foo -o type=registry
 ```
 
+> **Note **
+>
+> Since BuildKit v0.13.0 multiple outputs can be specified by repeating the flag.
+
 Supported exported types are:
 
 #### `local`