driver: automatically export image to MobyDriver

Signed-off-by: Tibor Vass <tibor@docker.com>

build/build.go

@@ -51,6 +51,9 @@ func Build(ctx context.Context, drivers []driver.Driver, opt map[string]Options,
 
 	pwOld := pw
 	d := drivers[0]
+	_, isDefaultMobyDriver := d.(interface {
+		IsDefaultMobyDriver()
+	})
 	c, pw, err := driver.Boot(ctx, d, pw)
 	if err != nil {
 		close(pwOld.Status())
@@ -75,7 +78,16 @@ func Build(ctx context.Context, drivers []driver.Driver, opt map[string]Options,
 			FrontendAttrs: map[string]string{},
 		}
 
-		if len(opt.Exports) > 1 {
+		switch len(opt.Exports) {
+		case 1:
+			// valid
+		case 0:
+			if isDefaultMobyDriver {
+				// backwards compat for docker driver only:
+				// this ensures the build results in a docker image.
+				opt.Exports = []client.ExportEntry{{Type: "image", Attrs: map[string]string{}}}
+			}
+		default:
 			return nil, errors.Errorf("multiple outputs currently unsupported")
 		}
 
@@ -100,18 +112,21 @@ func Build(ctx context.Context, drivers []driver.Driver, opt map[string]Options,
 			if e.Type == "oci" && !d.Features()[driver.OCIExporter] {
 				return nil, notSupported(d, driver.OCIExporter)
 			}
-			if e.Type == "docker" && e.Output != nil && !d.Features()[driver.DockerExporter] {
-				return nil, notSupported(d, driver.DockerExporter)
+			if e.Type == "docker" {
+				if e.Output == nil {
+					if !isDefaultMobyDriver {
+						return nil, errors.Errorf("loading to docker currently not implemented, specify dest file or -")
+					}
+					e.Type = "image"
+				} else if !d.Features()[driver.DockerExporter] {
+					return nil, notSupported(d, driver.DockerExporter)
+				}
 			}
-			if e.Type == "image" {
-				if _, ok := d.(interface {
-					IsDefaultMobyDriver()
-				}); ok {
-					opt.Exports[i].Type = "moby"
-					if e.Attrs["push"] != "" {
-						if ok, _ := strconv.ParseBool(e.Attrs["push"]); ok {
-							return nil, errors.Errorf("auto-push is currently not implemented for moby driver")
-						}
+			if e.Type == "image" && isDefaultMobyDriver {
+				opt.Exports[i].Type = "moby"
+				if e.Attrs["push"] != "" {
+					if ok, _ := strconv.ParseBool(e.Attrs["push"]); ok {
+						return nil, errors.Errorf("auto-push is currently not implemented for moby driver")
 					}
 				}
 			}

build/output.go

@@ -64,8 +64,6 @@ func ParseOutputs(inp []string) ([]client.ExportEntry, error) {
 			if !ok {
 				if out.Type != client.ExporterDocker {
 					dest = "-"
-				} else {
-					return nil, errors.Errorf("loading to docker currently not implemented, specify dest file or -")
 				}
 			}
 			if dest == "-" {