Improve section about docker for testing jobs

Context

Thanks to the meeting with Alexia and Valentin, we point out some case that need to be specified in the documentation. According with Thibault, we should improve the section Testing by adding more infos on how to test jobs with docker.

Objective

Specify how to mount an image with interactive shell and test commands one by one :
docker run -v $PWD:/$IMG_PWD -it <image> sh After testing, create the job and test it into a pipeline