Fix a bug in snapshotting with multi-stage images. (#546)

We previously had an optimization that would skip snapshotting mutli-stage images
when in an intermediate stage, until the very end.

This conflicted with another optimization to avoid snapshotting when no files had changed.

pkg/executor/build.go

@@ -309,11 +309,6 @@ func (s *stageBuilder) shouldTakeSnapshot(index int, files []string) bool {
 		return true
 	}
 
-	// We only snapshot the very end of intermediate stages.
-	if !s.stage.Final {
-		return isLastCommand
-	}
-
 	// nil means snapshot everything.
 	if files == nil {
 		return true

pkg/executor/build_test.go

@@ -147,7 +147,7 @@ func Test_stageBuilder_shouldTakeSnapshot(t *testing.T) {
 			args: args{
 				index: 0,
 			},
-			want: false,
+			want: true,
 		},
 		{
 			name: "caching enabled intermediate container",