@nebulon Another try hopefully less hackish... According to the solution of rLoutrel found here ... maybe it is sufficient to simply:

RUN ln -s /app/data/fonts /opt/collaboraoffice/share/fonts/truetype/local

This one won't add /usr/share/fonts/ as extra volume to collabora and we won' have the ability to get rid of the intially installed fonts but should make it at least possible to add custom fonts via /app/data/fonts. So this might be a solution to the initial Question of SirFiChi...