Add container image with Podman for use in runner

[detjensrobert]

This PR adds a new container build that includes Podman for container-in-container use. Podman does not require a daemon and as such is easier to manage inside an exising container.

The new image builds off the existing onecommons/unfurl image, and follows the existing tagging scheme (latest / stable / TAG), with a -podman suffix (latest-podman / stable-podman / TAG-podman)

Also includes a slimming of the exising image by ~200MB by clearing the pip cache, and typo fixes in the README and an example on using the Docker image.

Resolves onecommons/gitlab-oc#968.

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