Init containers can be used to perform initialization logic before the main containers run, these might include

• Waiting for a service to become available
• Run database migrations
• Copying files to a shared location
• Configuration set-up

Init containers must run sequentially and complete successfully.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 1
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      initContainers:
      - name: wait-for-db
        image: busybox
        command: ['sh', '-c', 'until nc -z db-service 5432; do echo waiting; sleep 2; done']
      containers:
      - name: app
        image: my-app-image
        ports:
        - containerPort: 8080

This waits for a PostgreSQL service to become available before our application can start.