Changed the image registry from the Docker hub to the CERN gitlab in … #94

Job	Run time
Build base images (centos7)	3m 30s
Build base images (centos9)	4m 32s
Build base images (cuda)	10m 8s
Build base images (jupyter)	8m 32s
Build workflow images (centos7, tf)	1m 18s
Build workflow images (centos9, tf)	1m 13s
Build workflow images (cuda, tf)	6m 17s
Build workflow images (jupyter, tf)	6m 22s
Build final images (centos7, tf, uc)	1m 40s
Build final images (centos7, tf, bnl)	1m 19s
Build final images (centos7, tf, slac)	1m 13s
Build final images (centos9, tf, uc)	1m 18s
Build final images (centos9, tf, bnl)	1m 28s
Build final images (centos9, tf, slac)	1m 12s
Build final images (cuda, tf, uc)	7m 30s
Build final images (cuda, tf, bnl)	23m 5s
Build final images (cuda, tf, slac)	6m 40s
Build final images (jupyter, tf, uc)	6m 36s
Build final images (jupyter, tf, bnl)	4m 45s
Build final images (jupyter, tf, slac)	6m 43s
	1h 45m 21s

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