support Podman for local development

[skriss]

Some contributors may want to use Podman locally to run containers. The scripts and documentation should be updated to support this use case in addition to the current use case of using Docker. From my initial testing, here are some things that need to be addressed:

• use podman command instead of docker in scripts (e.g. https://github.com/projectcontour/contour/blob/main/test/scripts/make-kind-cluster.sh, make container command)
• set KIND_EXPERIMENTAL_PROVIDER=podman
• the format of the podman network inspect output is different than that of docker network inspect, which breaks https://github.com/projectcontour/contour/blob/main/test/scripts/make-kind-cluster.sh#L78-L89 -- update for the podman format
• kind load docker-image does not work when using Podman -- need to replace with podman save image | kind load image-archive - (ref. https://github.com/projectcontour/contour/blob/main/test/scripts/install-contour-working.sh#L37, load: kind load docker-image .. is non-functional for provider podman kubernetes-sigs/kind#2729 (comment))

Ultimately, running commands like make install-contour-release and make install-contour-working should work properly with Podman.