If you don't have access to the docker host that sounds like a toll order. From my understanding, You want to run Ubuntu 20.04 container and do away with CentOS 7, that is more like running a whole new container in the docker host. If you mean running Ubuntu 20.04 inside the CentOS 7...one piece of advice, DON'T! But if you feel explorative, you can install docker engine inside the CentOS 7 then run an Ubuntu 20.04. You could theoretically also use LXC/LXD or a similar lightweight virtualization inside a Docker container to simulate nesting, but this is complex and error-prone.