add background-worker.sh script for nextcloud #81

Job	Run time
Build App Containers (minecraft) / Build minecraft	18s
Build App Containers (indico) / Build indico	2m 10s
Build App Containers (limesurvey) / Build limesurvey	34s
Build App Containers (etherpad) / Build etherpad	39s
Build App Containers (nextcloud) / Build nextcloud	1m 32s
Build App Containers (sogo) / Build sogo	2m 44s
	7m 57s

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