Creating singularity builder image for templates

[bbarker]

This will be somewhat meta, as ideally we should be able to just build these images on any system with docker and/or singularity, but we're starting to see some dependencies in the build process:

Here is a list (update as necessary):

• spython (from singularity-cli) - used occasionally to generate Singularity recipes from Dockerfiles. Probably not necessary to include, but might as well if we build this.
• GNU gettext (for envsubst) - used while Singularity doesn't support environment variable expansion.