Singularity to Apptainer renaming
Singularity has been renamed to Apptainer in 2022 due to legal constraints. In general, just the name has changed and all options are the same. Replacing the command `singularity` with `apptainer` should work on updated systems.
Since building a Apptainer/Singularity container requires `root` capabilities or a uid/gid mapping, it might be easier to use Gitlab at https://gitlab.desy.de to build your containers there.
Please see for details https://gitlab.desy.de/thomas.hartmann/singularitybuilder