test.sh

kubectl create -f test.yaml
#secret "postgres-files-secret" created
#statefulset "postgres-statefulset" created

kubectl exec -it postgres-statefulset-0 /bin/sh

ls -lah /var/lib/postgresql/
# total 12
# drwxr-xr-x    3 root     root        4.0K May 19 16:02 .
# drwxr-xr-x    3 root     root        4.0K May 19 16:02 ..
# drwxrwxrwx    2 root     root        4.0K May 19 16:02 data

ls /var/lib/postgresql/data/
# 

test.yaml

apiVersion: v1
kind: Secret
metadata:
  name: postgres-files-secret
type: Opaque
data:
  postgresql.conf: Zm9v
  pg_hba.conf: Zm9v

---

kind: StatefulSet
apiVersion: apps/v1beta1
metadata:
  name: postgres-statefulset
spec:
  serviceName: "postgres"
  replicas: 1
  template:
    metadata:
      labels:
        app: postgres
      annotations:
        pod.alpha.kubernetes.io/initialized: "true"
    spec:
      imagePullSecrets:
        - name: regsecret
      volumes:
        -
          name: postgres-files-secret
          secret:
            secretName: postgres-files-secret
            items:
              -
                key: postgresql.conf
                path: "./postgresql.conf"
                mode: 0777
              -
                key: pg_hba.conf
                path: "./pg_hba.conf"
                mode: 0777
        -
          name: postgresdata
          emptyDir: {}
      containers:
        -
          name: postgres
          image: tomaskral/sleeper
          ports:
            -
              name: postgres
              containerPort: 5432
          volumeMounts:
            -
              name: postgresdata
              mountPath: /var/lib/postgresql/data
            -
              name: postgres-files-secret
              readOnly: true
              mountPath: /etc/postgresql/