Optitype for Singularity

I am trying to use optitype on an HPC without docker support or sudo privileges. A search brought me https://github.com/researchapps/optitype and https://github.com/nf-core/hlatyping. I tried bootstrapping the singularity image as follows:

singularity build optitype.simg docker://fred2/optitype

However, this does not seem to run as expected. Could someone with experience bootstrapping docker files onto singularity suggest how I could change this? How do the commands change when doing a bootstrap? I would appreciate your help!