docker: stop on fatal errors while building

[diegodelemos]

Stems from reanahub/reana-job-controller#283 (review)

In reanahub/reana-job-controller#283 we spotted that an important instruction to build the docker image was failing but the image was being built "successfully". This is dangerous because we can end up with an image that doesn't fulfill the necessary requirements.

In this case we used set -e. We should runa campaign across REANA repos to find instructions like this ones that should exit on failure (w.g. wget).