Prebuilt image enhancements #6

Job	Run time
build (docker_open5gs, base, Dockerfile)	4m 32s
build (docker_kamailio, ims_base, Dockerfile)	3m 1s
build (docker_srslte, srslte, Dockerfile)	15m 16s
build (docker_srsran, srsran, Dockerfile)	31m 35s
build (docker_ueransim, ueransim, Dockerfile)	5m 7s
build (docker_dns, dns, Dockerfile)	21s
build (docker_mysql, mysql, Dockerfile)	44s
build (docker_pyhss, pyhss, Dockerfile)	1m 17s
build (docker_osmomsc, osmomsc, Dockerfile)	25s
build (docker_osmohlr, osmohlr, Dockerfile)	26s
build (docker_metrics, metrics, Dockerfile)	22s
build (docker_grafana, grafana, Dockerfile)	45s
build (docker_rtpengine, rtpengine, Dockerfile)	3m 29s
build (docker_oai_enb, oai, enb_dockerfile)	27m 51s
build (docker_oai_gnb, oai, gnb_dockerfile)	33m 13s
	2h 8m 24s

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