This MR introduces native
.sif store at registry.cern.ch
Currently, pushing using
singularity push requires singularity>=3.8, not yet available on EPEL.
workaround using ORAS (the registry tool included with singularity)
Cleaned up some caching logic - no need to tag;push;pull;rebuild;remove tags;remove images - just simply build .sif from already local docker daemon. delete when done testing (& pushing).