build: when building multiple targets include name in error

Some errors can appear without a stacktrace or progress record,
eg. wrong Dockerfile name passed. In that case when building many
targets with bake it might be hard to figure out which target
failed as in the progressbar there will only be steps that
were cancelled.

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

build/build.go

@@ -309,7 +309,7 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 	childTargets := calculateChildTargets(reqForNodes, opt)
 
 	for k, opt := range opt {
-		err := func(k string) error {
+		err := func(k string) (err error) {
 			opt := opt
 			dps := drivers[k]
 			multiDriver := len(drivers[k]) > 1
@@ -321,6 +321,12 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 			}
 			baseCtx := ctx
 
+			if multiTarget {
+				defer func() {
+					err = errors.Wrapf(err, "target %s", k)
+				}()
+			}
+
 			res := make([]*client.SolveResponse, len(dps))
 			eg2, ctx := errgroup.WithContext(ctx)
 
@@ -563,6 +569,13 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 						tracing.FinishWithError(span, err)
 					}
 				}()
+
+				if multiTarget {
+					defer func() {
+						err = errors.Wrapf(err, "target %s", k)
+					}()
+				}
+
 				pw := progress.WithPrefix(w, "default", false)
 				if err := eg2.Wait(); err != nil {
 					return err