Consider adding support for apptainer/singularity

[juliasilge]

We already have support for generating Dockerfiles plus needed artifacts for serving vetiver models. Let's consider also adding support for Apptainer/Singularity containers. Apptainer/Singularity is different from Docker in that a data scientist could build and run a container completely in user space (without admin or close to admin privileges); this may be a better option for more constrained computational environments. We may not need to change much to add this support, since the interfaces are so similar.

I notice the rang package has support for both and would be worth checking out in more detail.