Push .sif to registry.cern.ch/hep-workloads

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).