Install mongoexport in surveyadmin deployment

It seems that relying on a custom CLI tool or binary requires a custom Docker image on Render.com.

- [ ] Try to get binaries built for mongoexport? Not sure it can work, but see https://community.render.com/t/install-custom-packages-on-my-deploy-do-i-need-docker/5857
- [ ] Otherwise try deployment with a custom Dockerfile => almost there but somehow the file structure of the end image is wrong. Anyway a Docker image on a monorepo will be very bristle
- [ ] Add mongoexport to the Dockerfile

Current process:
- Run the app locally, but with MONGO_URI aiming at the prod db (careful on that)
- Connect as admin **/!\ when connecting via anonymous login, it will find based on emailHash, but via password login, via pure email** => I need to clean this part up a bit
- Export from your own machinge