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).

.gitlab-ci.yml

@@ -47,7 +47,7 @@ debian_oldstable_py38:
   before_script:
     - pyenv global 3.8
   stage: build
-  script: *docker_build
+  script: &docker_build
     - ORIGINAL_DIR=$(pwd)
     - export CC=gcc; export CXX=g++
     - export MPLBACKEND=Agg