Document podman based builds

🔥 Problem

In !201 (merged), an experimental feature was added: use podman to build the application image. podman provides benefits such as the build being a rootless operation.

This feature is still flagged as experimental. However, we can start documenting it.

🚒 Solution

Add a new section in https://docs.gitlab.com/topics/autodevops/stages/#auto-build about this podman build and provide instructions into how to use it.

Make it extra clear that this feature is experimental.