fix background-worker.sh su command #83

Job	Run time
Build App Containers (indico) / Build indico	2m 19s
Build App Containers (etherpad) / Build etherpad	43s
Build App Containers (limesurvey) / Build limesurvey	46s
Build App Containers (minecraft) / Build minecraft	41s
Build App Containers (nextcloud) / Build nextcloud	1m 28s
Build App Containers (sogo) / Build sogo	2m 54s
	8m 51s

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