TL/DR: at this moment, I’m now aware about any implicit way how to let users install extensions into system-provided Slicer.
longer answer:
Hi, (I see nobody answered yet your question, so giving my 5c)
first, I’m more Apptainer guy rather than Slicer expert - we’re investigating how to use Slicer properly at our site too.
Did you try to build SlicerRadiomics in %post section of apptainer definition file, to have extension baked in apptainer image?
Cleaner way could be to install base slicer, and then, create apptainer image derived from base Slicer image with installed extension(s)…
The reason behind is (it was explained here at this forum recently) that Slicer mixes application itself and user-installed extensions in its “root” directory…