feat: use dockerhub through internal mirror (#1007)

# Why:
pull from dockerhub directly will
- use internet bandwidth
- meet 'TOOMANYREQUESTS: You have reached your pull rate limit'

# How:
- pull dockerhub image through PingCAP internal mirror

---------

Signed-off-by: lijie <lijie@pingcap.com>

apps/prod/tekton/configs/tasks/pingcap-build-images.yaml

@@ -75,6 +75,8 @@ spec:
       env:
         - name: KANIKO_EXECUTOR
           value: /kaniko/executor
+        - name: KANIKO_REGISTRY_MIRROR
+          value: registry-mirror.pingcap.net
       resources:
         requests:
           cpu: "4"