以下を実行するだけ！！

cat <<EOF | kubectl apply -f-
apiVersion: v1
kind: Secret
metadata:
  name: postgres-password
type: Opaque
data:
  password: dGVzdA==
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: postgres
spec:
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
      - name: postgres
        image: postgres:13-alpine
        env:
        - name: POSTGRES_PASSWORD
          valueFrom:
            secretKeyRef:
              name: postgres-password
              key: password
        ports:
        - containerPort: 5432
---
apiVersion: v1
kind: Service
metadata:
  name: postgres
spec:
  selector:
    app: postgres
  ports:
  - port: 5432
    targetPort: 5432

postgresへのログイン方法

$ kubectl get po | grep postgres | awk '{print $1}' 

$ kubectl exec -it [表示されたpostgres pod] sh
$ psql -U posgres
# postgres