Add apptainer container. Clean up repo

Singularity.def

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+Bootstrap: docker
+From: python:3.11-slim
+
+%post
+    apt-get update
+    apt-get install -y --no-install-recommends imagemagick
+    rm -rf /var/lib/apt/lists/*
+
+    pip install --no-cache-dir jinja2 pyyaml
+    mkdir -p /src
+
+%files
+    . /src
+
+%environment
+    export PYTHONPATH=/src
+
+%runscript
+    cd /src
+    exec python3 generate_gallery.py "$@"