```console
$ docker run -it --rm mono:onbuild ls -lAFh /usr/src/app/
drwxr-xr-x 1 root root 0 Jan 14 21:09 source/
drwxr-xr-x 1 root root 0 Jan 1 07:44 {source,build}/
```
Also, we don't save any space by removing /usr/src/app/source since it's already baked into a layer of the image, so we might as well just leave it.