"Singularity containers can be used to package entire scientific workflows, software and libraries, and even data. This means that you don’t have to ask your cluster admin to install anything for you - you can put it in a Singularity container and run" - Singularity website
"Singularity containers let users run applications in a Linux environment of their choosing" -- BioWulf User Guide
"Singularity is a container system for HPC that lets you define your own environment and makes your work portable and reproducible on any HPC that supports it" -- C3SE User Guide
Short answer: Singularity is the only solution that we can install without major system changes or creating too many security problems.
We believe that Singularity will solve many of the problems in getting certain applications to run with a very low cost to NSC and users.
This blog post has a comparison between some Docker, Singularity and Shifter.