Multi-stage cached docker build issues

Following discussions here, I want to suggest a new arch for the docker thing (without multistage build).
Thus allowing us to rebuild TF only when necessary.
The general idea is to use a base image of TF which is stored in subdir of the repo (like gitlab-registry.irstea.fr/remi.cresson/otbtf/base ) where every base of different TF build are stored (tags like tf-2.8.0-cpu-avx2). Then there is a second Dockerfile where the arg BASE_IMG is one of the TF base image. Finally it is pushed at the root of the repo registry.
This should ease the process of building and caching during a CI job.