Temporarily use root user, run sudo ipykernel install system wide, reset to previous user (1000:100)
USER 0:0
RUN sudo "${CONDA_DIR}/envs/${conda_env}/bin/python" -m ipykernel install --name="${conda_env}" && \
fix-permissions "${CONDA_DIR}" && \
fix-permissions "/home/${NB_USER}"
USER 1000:100