Re-writing the [gitian-build.sh](https://github.com/bitcoin/bitcoin/blob/master/contrib/gitian-build.sh) has come up multiple times, most recently in [here](https://github.com/bitcoin/bitcoin/pull/13609#issuecomment-403470887). Opening a "good first issue" so someone can find it.