Use multi-stage builds in Dockerfiles

[NoeTerrier]

Copy from issue clicepfl/website#36 of CLIC website:

Should we use multi-stage builds?

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 clicepfl/website#33 (comment)_

Other links:

• why use root as the default user? nodejs/docker-node#1169
• https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#non-root-user