Use multi-stage builds in Dockerfiles

Copy from issue https://github.com/clicepfl/website/issues/36 of CLIC website:

>
> Should we use [multi-stage builds](https://docs.docker.com/build/building/multi-stage/)?
>
> This would improve the size and the security of the images, because the build tools would stay in the builder step image. Then we only transfer the required compiled assets and have a final image that is minimal and that only contains the runtime and the executed code.
>
> See original discussion in https://github.com/clicepfl/website/pull/33#discussion_r1281900621_
>
> Other links:
> - https://github.com/nodejs/docker-node/issues/1169
> - https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#non-root-user