Print commitID with --layers

Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

When the --layers option is used, the final imageID was
not being printed in the final line.  It was printed two
lines prior.  We still have to print the one two lines up
(at least without major modifications), but now we print
it in the last line as it does for all other circumstances
so it can be grabbed by scripts.

Fixes: containers#1237

Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

imagebuildah/build.go

@@ -809,6 +809,7 @@ func (b *Executor) Execute(ctx context.Context, stage imagebuilder.Stage) error
 	b.containerIDs = nil
 
 	var leftoverArgs []string
+	var layerCacheID string
 	for arg := range b.builder.Args {
 		if !builtinAllowedBuildArgs[arg] {
 			leftoverArgs = append(leftoverArgs, arg)
@@ -892,6 +893,7 @@ func (b *Executor) Execute(ctx context.Context, stage imagebuilder.Stage) error
 				return err
 			}
 			b.containerIDs = append(b.containerIDs, b.builder.ContainerID)
+			layerCacheID = cacheID
 			break
 		}
 
@@ -910,6 +912,7 @@ func (b *Executor) Execute(ctx context.Context, stage imagebuilder.Stage) error
 				return errors.Wrapf(err, "error committing container for step %+v", *step)
 			}
 			if i == len(children)-1 {
+				layerCacheID = imgID
 				b.log("COMMIT %s", b.output)
 			}
 		} else {
@@ -929,6 +932,10 @@ func (b *Executor) Execute(ctx context.Context, stage imagebuilder.Stage) error
 	if len(leftoverArgs) > 0 {
 		fmt.Fprintf(b.out, "[Warning] One or more build-args %v were not consumed\n", leftoverArgs)
 	}
+
+	if b.layers { // print out the final imageID if we're using layers flag
+		fmt.Fprintf(b.out, "--> %s\n", layerCacheID)
+	}
 	return nil
 }