Fix docker build in CI

The CI became broken due to CERN's migration to the kaniko-based docker-image-builder service in earlier October: https://cern.service-now.com/service-portal?id=outage&n=OTG0078219

Also see this relevant PATHelp email thread: https://groups.cern.ch/group/hn-atlas-PATHelp/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DPATHelp%2FLists%2FArchive%2FFailing%20docker%20images%20in%20GitLab%20CI%20pipelines&FolderCTID=0x0120020084D7E80CB0C3394A8D54EF07C309B044