docker: Use is-active to determine docker status
    
    Only run `systemctl start docker` when `systemctl is-active docker`
    fails. The start command is privileged and therefore requires manual
    authentication if it is run by a non-root user even if the service is
    already active. The check with is-active is possible without special
    privileges. This allows to use the inventory file if the current user is
    in the docker group.