Update Dockerfile to benefit build cache

Update Dockerfile to benefit build cache

Docker has recently introduces buikld-stages (as of version 17.05)

Build stages allows to benefit the docker build cache as well as
reducing the size of the resulting image c.f.
https://docs.docker.com/engine/userguide/eng-image/multistage-build/

This change allows to have faster builds when running `docker build`
several times after changing some little code

Signed-off-by: Thibault Jamet <tjamet@users.noreply.github.com>