build: set same ref id when building on multiple nodes

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

build/build.go

@@ -32,6 +32,7 @@ import (
 	"github.com/moby/buildkit/client/llb"
 	"github.com/moby/buildkit/exporter/containerimage/exptypes"
 	gateway "github.com/moby/buildkit/frontend/gateway/client"
+	"github.com/moby/buildkit/identity"
 	"github.com/moby/buildkit/session"
 	"github.com/moby/buildkit/solver/errdefs"
 	"github.com/moby/buildkit/solver/pb"
@@ -216,6 +217,9 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 		if err != nil {
 			logrus.WithError(err).Warn("current commit information was not captured by the build")
 		}
+		if opt.Ref == "" {
+			opt.Ref = identity.NewID()
+		}
 		var reqn []*reqForNode
 		for _, np := range drivers[k] {
 			if np.Node().Driver.IsMobyDriver() {