e2e tests: run with podman as well as docker

[krishnapermi]

Closes #52

Adds podman as a second container runtime to the end-to-end test matrix, alongside the existing docker jobs.

Changes

• Add runtime: [docker, podman] as a new matrix dimension in endtoend_tests.yml
• Add a "Configure podman to allow the local registry" step (runs only when matrix.runtime == 'podman') that registers localhost:5000 as an insecure registry in /etc/containers/registries.conf.d/. Podman rejects plain-HTTP registries by default, so this step is necessary for the local registry:2 service used in CI.
• Replace the hardcoded docker run in the "Run the example" step with ${{ matrix.runtime }} run, so the same step exercises both runtimes.

The step name also includes ${{ matrix.runtime }} to make it easy to distinguish docker vs podman runs in the Actions UI.

I spotted this while looking through the open issue list after submitting previous PRs. Podman is pre-installed on ubuntu-latest, and since the test already pushes to a local registry and pulls from it at run time, switching the pull/run step to use podman should be a faithful compatibility check with no other changes needed.

[euanh]

It looks like this is an older configuration format which doesn't match the rest of the podman configuration:

Error: loading drop-in registries configuration "/etc/containers/registries.conf.d/local-registry.conf": registry must be in v2 format but is in v1