Add how to mount a specific key in configmap/secret to container directory

[life1347]

apiVersion: v1
kind: ConfigMap
metadata:
  name: core-env
data:
  .foo: |
    APP=xxx
  .bar: |
    APP=xxx

        - mountPath: /var/www/html/.foo
          name: env-config
          subPath: .foo
      volumes:
      - name: env-config
        configMap:
          name: core-env
          defaultMode: 420

此處只會將 configmap 內的 .foo key 的內容掛載進去
和下面的方式主要差異在於，上面是將內容直接以檔案形式放進去，不會影響到 /var/www/html/ 裡的內容
下面為將 volume 掛在 /etc/config，此方式會導致原先在 /etc/config 內的東西消失

apiVersion: v1
kind: Pod
metadata:
  name: configmap-pod
spec:
  containers:
    - name: test
      image: busybox
      volumeMounts:
        - name: config-vol
          mountPath: /etc/config
  volumes:
    - name: config-vol
      configMap:
        name: log-config
        items:
          - key: log_level
            path: log_level