Optitype for Singularity
Bit of an old post sorry, but I think you might be getting a little confused here. The nf-core pipelines come with their own software already, so you don't need to install or bootstrap optitype yourself. If it's used in the pipeline, it comes in the pipeline container(s) (or conda environment or whatever packaging system you use). So you should never have to touch docker://fred2/optitype.
When you run the pipeline with the -profile singularity argument, it will automatically pull the image from Docker Hub and convert to Singularity for you (some of the newer pipelines pull pre-build Singularity images directly). You can also use the nf-core download hlatyping --singularity command from the helper tools to pull the image ahead of time.

Docker images for this pipeline: https://hub.docker.com/r/nfcore/hlatyping
Documentation for this pipeline: https://nf-co.re/hlatyping/
nf-core/tools download docs: https://nf-co.re/tools/#downloading-pipelines-for-offline-use

I hope this helps! Come and join the nf-core Slack organisation if you need more help. We have a channel for each pipeline, including #hlatyping.