Add a bit more context to layer offset failures

In GoogleContainerTools#251 we are investigating test flakes due to layer offsets not
matching, this change will give us a bit more context so we can be sure
which image has which number of layers.

Also updated reproducible Dockerfile to be built with reproducible flag,
which I think was the original intent (without this change, there is no
difference between how `kaniko-dockerfile_test_copy_reproducible` and
`kaniko-dockerfile_test_copy` are built.

integration/images.go

@@ -63,7 +63,7 @@ var additionalKanikoFlagsMap = map[string][]string{
 }
 
 var bucketContextTests = []string{"Dockerfile_test_copy_bucket"}
-var reproducibleTests = []string{"Dockerfile_test_env"}
+var reproducibleTests = []string{"Dockerfile_test_reproducible"}
 
 // GetDockerImage constructs the name of the docker image that would be built with
 // dockerfile if it was tagged with imageRepo.

integration/integration_test.go

@@ -225,7 +225,7 @@ func checkLayers(image1, image2 string, offset int) error {
 	}
 	actualOffset := int(math.Abs(float64(lenImage1 - lenImage2)))
 	if actualOffset != offset {
-		return fmt.Errorf("incorrect offset between layers of %s and %s: expected %d but got %d", image1, image2, offset, actualOffset)
+		return fmt.Errorf("Difference in number of layers in each image is %d but should be %d. %s has %d layers and %s has %d layers", actualOffset, offset, image1, lenImage1, image2, lenImage2)
 	}
 	return nil
 }