You can use Kubernetes postStart lifecycle hook to create your schema on given Postgres DB. Just update your deployment file with the lifecycle block.

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: postgres
  name: postgres
spec:
  containers:
  - image: postgres:10.4
    name: postgres
    resources: {}
    lifecycle:
      postStart:
        exec:
          command: ["/bin/bash","-c","sleep 20 && PGPASSWORD=$POSTGRES_PASSWORD psql $POSTGRES_DB -U $POSTGRES_USER -c \'CREATE SCHEMA IF NOT EXISTS key;\'"]
    envFrom:
    - configMapRef:
        name: postgres-config
    volumeMounts:
    - mountPath: /var/lib/postgresql/data
      name: data
  dnsPolicy: ClusterFirst
  restartPolicy: Never
  volumes:
  - name: data
    emptyDir: {}
status: {}