docker: easier shared-module building
docker: easier shared-module building

* Connects reanahub/reana#132.

Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

diegodelemos/reana-job-controller,tiborsimko/reana-job-controller,reanahub/reana-job-controller,tiborsimko/reana-job-controller,reanahub/reana-job-controller,diegodelemos/reana-job-controller