CI build procedure rework
This is a major rework of the build procedure implemented in the CI, intended to
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partition the monolithic CI process, in order to speed up the build process and avoid re-executions of successful steps in case of failures in test phases or push to registry phases.
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implement the parallel built of images for multiple architectures, starting from the same Experiment/Workload directory. Therefore there is not need to maintain separate Experiment/Workload directories for x86, aarch64, etc.
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improve the traceability, tagging the repository with a tag associated to the image built, in order to recover more easily the code that generated a given production container (i.e. tagged with v* version)
The old main is in the backup branch master_legacy and tag v1.0_legacy.
In order to enable these changes, the builder image has been upgraded to centos:8, in order to use the singularity rpm for ARM. The new image name is hep-workload-builder-multiarch. The legacy hep-workload-builder still stays in the registy