Debian/oldstable docker container for packaging (#942)

This adds dockerized build stances to .gitlab-ci.yml. They will run only in scheduled builds (i.e. once per day at night) or upon special request (through secret GUI button). They shall ensure that at any time we are ready to generate binary installers.

This resolves #942 (closed).

Wrap/Python/src/bornagain_c_ext_dummy.c

-// dummy C extension to set the correct wheel tag according to PEP 425
-//*** Please DO NOT change the contents ***//
-
-#include <Python.h>
-
-PyMODINIT_FUNC PyInit___bornagain_dummy_c_ext(void)
-{
-    return NULL;
-}
-// END