Specify memory limits as strings for docker compose
    
    Fixes the following error in Azure Pipelines CI after upgrade of Docker
    setup:
    
    [2020-09-14 06:50:07] The Compose file './docker-compose.yml' is invalid because:
    [2020-09-14 06:50:07] services.client.mem_limit contains an invalid type, it should be a string
    
    Fixes: https://pagure.io/freeipa/issue/8494
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Levin <slev@altlinux.org>