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).

devtools/deploy/linux/Dockerfile.debian.oldstable

+# update: docker build -t scg-debian-oldstable:latest -f ~/docker/Dockerfile.debian.oldstable .
+
+FROM debian:oldstable
+
+RUN echo "deb http://deb.debian.org/debian bullseye-backports main" >> /etc/apt/sources.list
+RUN apt-get update
+RUN apt-get install -y \
+  cmake/bullseye-backports cmake-data/bullseye-backports \
+  libgsl-dev libfftw3-dev libtiff5-dev \
+  libboost-dev libboost-iostreams-dev \
+  libboost-program-options-dev libboost-regex-dev \
+  libcerf-dev qt6-base-dev/bullseye-backports qt6-svg-dev/bullseye-backports \
+  curl git tk-dev ccache xvfb \
+  zlib1g-dev libffi-dev libssl-dev libbz2-dev libreadline-dev libsqlite3-dev liblzma-dev
+
+# auditwheel requires patchelf >= 0.14; Debian/oldstable only has 0.12.
+RUN git clone https://github.com/NixOS/patchelf.git && cd patchelf && ./bootstrap.sh && ./configure && make && make check && make install
+
+ENV HOME /root
+RUN curl https://pyenv.run | bash
+ENV PYENV_ROOT="/root/.pyenv"
+ENV PATH="$PYENV_ROOT/bin:$PATH"
+RUN pyenv install 3.8 3.9 3.10 3.11
+RUN eval "$(pyenv init -)" && pyenv global 3.8  && pip install --upgrade pip && pip install numpy matplotlib corner emcee lmfit scipy tqdm wheel auditwheel
+RUN eval "$(pyenv init -)" && pyenv global 3.9  && pip install --upgrade pip && pip install numpy matplotlib corner emcee lmfit scipy tqdm wheel auditwheel
+RUN eval "$(pyenv init -)" && pyenv global 3.10  && pip install --upgrade pip && pip install numpy matplotlib corner emcee lmfit scipy tqdm wheel auditwheel
+RUN eval "$(pyenv init -)" && pyenv global 3.11  && pip install --upgrade pip && pip install numpy matplotlib corner emcee lmfit scipy tqdm wheel auditwheel
+
+RUN git clone https://jugit.fz-juelich.de/mlz/libheinz.git
+RUN git clone https://jugit.fz-juelich.de/mlz/libcerf.git
+RUN git clone https://jugit.fz-juelich.de/mlz/libformfactor.git