Podman testing in CI

[mitchdenny]

Is now a good time to discuss Podman testing? At the moment all of our testing is done against Docker/Docker Desktop, but we say we support Podman but don't do any validation in our CI.

Originally posted by @mitchdenny in #5628 (comment)

At the moment we allow developers to select between Docker/Docker Desktop and Podman for container runtime support. However in our CI tests we only ever use Docker Desktop and I'd say 90% of our team uses Docker Desktop rather than Podman on a daily basis (the DCP folks probably swap between them a bit more than we do).

There are so many issues that we get filed where there are subtle compatibility issues/differences between Docker Desktop and Podman. A lot of these fall into container networking differences and we are making some investments there around setting up container networks (see: #5628).

This issue is more around how we can improve our test coverage for Podman moving forward. I think there are a few problems to solve here:

1. If we add test coverage for Podman (e.g. switch [Fact] to [Theory] to swapping container runtimes in tests) then developers will need to have both installed.
2. We will need Podman on build machines/helix agents.

We probably don't need to duplicate tests for both runtimes everywhere, there are probably just some key feature areas that we know need some coverage.

[mitchdenny]

/cc @davidfowl @danegsta @karolz-ms @joperezr @radical

[danegsta]

We could do two separate parallel test runs; one on a machine with Docker installed and the other on a machine with Podman. I've tried running some of the tests with just Podman Desktop active and they seem to mostly work.