builder/dockerfile/mockbackend_test.go: suppress SA9005 (staticcheck)

```
builder/dockerfile/mockbackend_test.go:107:21: SA9005: struct doesn't have any exported fields, nor custom marshaling (staticcheck)
	return json.Marshal(rawImage(*i))
	                   ^
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/mockbackend_test.go

@@ -104,7 +104,7 @@ func (i *mockImage) OperatingSystem() string {
 
 func (i *mockImage) MarshalJSON() ([]byte, error) {
 	type rawImage mockImage
-	return json.Marshal(rawImage(*i))
+	return json.Marshal(rawImage(*i)) //nolint:staticcheck
 }
 
 type mockImageCache struct {