Remotely building
Actually possible with Singularity account. More details at https://sylabs.io/guides/3.6/user-guide/cloud_library.html
singularity remote login
Create account and tokenfile at [https://cloud.sylabs.io/](Sylabs Cloud)
singularity remote login --tokenfile mytokenfile
Build an image
singularity build --remote focal.sif docker://ubuntu:focal