docs(helm): add example env required for docker-in-docker

Author: cjtim

ci/helm-chart/values.yaml

@@ -70,6 +70,8 @@ extraArgs: []
 extraVars: []
 #  - name: DISABLE_TELEMETRY
 #    value: true
+#  - name: DOCKER_HOST
+#    value: "tcp://localhost:2375"
 
 ##
 ## Init containers parameters:
@@ -126,6 +128,7 @@ persistence:
 ## Enable an Specify container in extraContainers.
 ## This is meant to allow adding code-server dependencies, like docker-dind.
 extraContainers: |
+# If docker-dind is used, DOCKER_HOST env is mandatory to set in "extraVars"
 #- name: docker-dind
 #  image: docker:19.03-dind
 #  imagePullPolicy: IfNotPresent