Ensure seamless integration with podman

[baronfel]

Podman supports a mode that exposes a Docker daemon that the Docker CLI can interact with. Our current feature set supports pushing to this daemon, but for some reason podman-specific commands like ps and image list don't return any images. docker equivalent commands do see the images, and the images can be run. We should investigate and see if there are metadata/configuration differences in the way these commands discover and execute images.

[baronfel]

Two hurdles that users may see:

• podman in rootless vs rootful mode stores images in different locations, so docker image list and podman image list may have inconsistent results
• podman prefixes registry-less image tags with localhost upon import: podman build tags images in the local registry with a leading localhost/ and docker does not containers/buildah#1034. this causes inconsistencies in user expectations, but won't be a problem if the user is using registry-included image names

[baronfel]

I'm going to keep this open for now - I think the action item would be to add a CI/test leg that uses podman instead of Docker and verify that the end-to-end tests all work.

[baronfel]

We have all of this now - Explicit Podman support in the tasks, and a CI run that uses podman.