builder/dockerfile: errmsg: quote build target

The build target is not quoted and it makes it difficult for some
persons to see what the problem is.

By quoting it we emphasize that the target name is variable.

Signed-off-by: Frank Villaro-Dixon <frank.villarodixon@merkle.com>

builder/dockerfile/builder.go

@@ -199,7 +199,7 @@ func (b *Builder) build(ctx context.Context, source builder.Source, dockerfile *
 		targetIx, found := instructions.HasStage(stages, b.options.Target)
 		if !found {
 			buildsFailed.WithValues(metricsBuildTargetNotReachableError).Inc()
-			return nil, errdefs.InvalidParameter(errors.Errorf("failed to reach build target %s in Dockerfile", b.options.Target))
+			return nil, errdefs.InvalidParameter(errors.Errorf("target stage %q could not be found", b.options.Target))
 		}
 		stages = stages[:targetIx+1]
 	}

integration-cli/docker_cli_build_test.go

@@ -5946,7 +5946,7 @@ func (s *DockerCLIBuildSuite) TestBuildIntermediateTarget(c *testing.T) {
 		cli.WithFlags("--target", "nosuchtarget"))
 	result.Assert(c, icmd.Expected{
 		ExitCode: 1,
-		Err:      "failed to reach build target",
+		Err:      "target stage \"nosuchtarget\" could not be found",
 	})
 }