Init container

A container that runs to completion before the main containers start, in order, used for setup: waiting on a dependency, running a schema migration, fetching configuration. The distinction from a sidecar is temporal — an init container finishes and the app then runs, a sidecar runs beside it — and confusing the two produces either a pod that never starts or a migration racing the application. If an init container fails, the pod restarts and runs it again, so it must be safe to repeat.