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

@@ -42,12 +42,12 @@ native_Debian:
   - ctest
   - make install
 
-debian_oldstable_py38:
+debian_oldstable_py311:
   tags:
     - LinuxDocker
   image: scg-debian-oldstable:latest
   before_script:
-    - pyenv global 3.8
+    - pyenv global 3.11
   stage: build
   script: &docker_build
     - export CHECK_FLAGS=""