Allow repository access in rootless CI containers
    
    GitLab CI clones the repository as root, but we prefer to use a
    regular user for some of our job specific containers.
    Add a helper script and call it before the respective script
    executions to work around the "dubious ownership" error as there
    does not seem to be a GitLab CI solution to clone as the target
    user.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>