this is the first pass at implementing init containers for podman pods.
init containersare made popular by k8s as a way to run setup for pods
before the pods standard containers run.
unlike k8s, we support two styles of init containers: always and
oneshot. always means the container stays in the pod and starts
whenever a pod is started. this does not apply to pods restarting.
oneshot means the container runs onetime when the pod starts and then is
removed.
Signed-off-by: Brent Baude <bbaude@redhat.com>
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