Prebuilt image enhancements #8

Job	Run time
build (docker_open5gs, base, Dockerfile)	4m 15s
build (docker_kamailio, ims_base, Dockerfile)	3m 26s
build (docker_srslte, srslte, Dockerfile)	14m 35s
build (docker_srsran, srsran, Dockerfile)	30m 58s
build (docker_ueransim, ueransim, Dockerfile)	4m 47s
build (docker_dns, dns, Dockerfile)	29s
build (docker_mysql, mysql, Dockerfile)	37s
build (docker_pyhss, pyhss, Dockerfile)	1m 15s
build (docker_osmomsc, osmomsc, Dockerfile)	29s
build (docker_osmohlr, osmohlr, Dockerfile)	26s
build (docker_metrics, metrics, Dockerfile)	20s
build (docker_grafana, grafana, Dockerfile)	47s
build (docker_rtpengine, rtpengine, Dockerfile)	3m 8s
build (docker_oai_enb, oai, enb_dockerfile)	25m 45s
build (docker_oai_gnb, oai, gnb_dockerfile)	25m 38s
	1h 56m 55s

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