Improvements to OAI buld and platform flexibility #15

Job	Run time
build (docker_open5gs, base, Dockerfile)	4m 7s
build (docker_kamailio, ims_base, Dockerfile)	3m 11s
build (docker_srslte, srslte, Dockerfile)	14m 6s
build (docker_srsran, srsran, Dockerfile)	29m 16s
build (docker_ueransim, ueransim, Dockerfile)	4m 34s
build (docker_dns, dns, Dockerfile)	32s
build (docker_mysql, mysql, Dockerfile)	43s
build (docker_pyhss, pyhss, Dockerfile)	1m 11s
build (docker_osmomsc, osmomsc, Dockerfile)	27s
build (docker_osmohlr, osmohlr, Dockerfile)	33s
build (docker_metrics, metrics, Dockerfile)	29s
build (docker_grafana, grafana, Dockerfile)	57s
build (docker_rtpengine, rtpengine, Dockerfile)	3m 20s
build (docker_oai_enb, oai, enb_dockerfile)	24m 58s
build (docker_oai_gnb, oai, gnb_dockerfile)	25m 21s
	1h 53m 45s

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