docs: Podman (rootful/rootless) usage docs (#987)

docs/usage/advanced/.pages

@@ -2,3 +2,4 @@ title: Advanced Guides
 nav:
   - calico.md
   - cuda.md
+  - podman.md

docs/usage/advanced/podman.md

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+# Using Podman instead of Docker
+
+Podman has an [Docker API compatibility layer](https://podman.io/blogs/2020/06/29/podman-v2-announce.html#restful-api). k3d uses the Docker API and is compatible with Podman v4 and higher.
+
+!!! important "Podman support is experimental"
+    k3d is not guaranteed to work with Podman. If you find a bug, do help by [filing an issue](https://github.com/rancher/k3d/issues/new?labels=bug&template=bug_report.md&title=%5BBUG%5D+Podman)
+
+## Using Podman
+
+Ensure the Podman system socket is available:
+
+```bash
+sudo systemctl enable --now podman.socket
+# or sudo podman system service --time=0
+```
+
+To point k3d at the right Docker socket, create a symbolic link:
+
+```bash
+ln -s /run/podman/podman.sock /var/run/docker.sock
+# or install your system podman-docker if available
+sudo k3d cluster create
+```
+
+Alternatively, set DOCKER_HOST when running k3d:
+
+```bash
+export DOCKER_HOST=unix:///run/podman/podman.sock
+sudo --preserve-env=DOCKER_HOST k3d cluster create
+```
+
+### Using rootless Podman
+
+Ensure the Podman user socket is available:
+
+```bash
+systemctl --user enable --now podman.socket
+# or podman system service --time=0
+```
+
+Set DOCKER_HOST when running k3d:
+
+```bash
+XDG_RUNTIME_DIR=${XDG_RUNTIME_DIR:-/run/user/$(id -u)}
+export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/podman/podman.sock
+k3d cluster create
+```
+
+## Creating local registries
+
+Because Podman does not have a default "bridge" network, you have to specify a network using the `--default-network` flag when creating a local registry:
+
+```bash
+k3d registry create --default-network podman mycluster-registry
+```
+
+To use this registry with a cluster, pass the `--registry-use` flag:
+
+```bash
+k3d cluster create --registry-use mycluster-registry mycluster
+```
+
+!!! note "Incompatibility with `--registry-create`"
+    Because `--registry-create` assumes the default network to be "bridge", avoid `--registry-create` when using Podman. Instead, always create a registry before creating a cluster.