update doc for podman (#3261)

* update doc for podman

* address comments

Author: Mengqi Yu

site/book/01-getting-started/01-system-requirements.md

@@ -13,9 +13,26 @@ $ kpt version
 
 kpt requires that you have [Git] installed and configured.
 
-## Docker
+## Container Runtime
 
-kpt requires that you have [Docker] installed and configured.
+`kpt` requires you to have at least one of the following runtimes installed and configured.
+
+
+
+### Docker
+
+Please follow the [instructions][install-docker] to install and configure Docker.
+
+### Podman
+
+Please follow the [instructions][install-podman] to install and configure Podman.
+
+If you want to set up rootless container runtime, [this][rootless] may be a
+useful resource for you.
+
+Environment variable can be used to control which container runtime to use. More
+details can be found in the reference documents for [`kpt fn render`](/reference/cli/fn/render/)
+and [`kpt fn eval`](/reference/cli/fn/eval/).
 
 ## Kubernetes cluster
 
@@ -24,6 +41,8 @@ In order to deploy the examples, you need a Kubernetes cluster and a configured
 For testing purposes, [kind] tool is useful for running ephemeral Kubernetes cluster on your local host.
 
 [install]: /installation/
-[docker]: https://docs.docker.com/get-docker/
+[install-docker]: https://docs.docker.com/get-docker/
+[install-podman]: https://podman.io/getting-started/installation
+[rootless]: https://rootlesscontaine.rs/
 [git]: https://git-scm.com/book/en/v2/Getting-Started-Installing-Git
 [kind]: https://kind.sigs.k8s.io/docs/user/quick-start/