message: update the build doc to show how to add customzied bazel options.
diff: @@ -70,10 +70,10 @@ The build artifact can be found in `/tmp/envoy-docker-build/envoy/source/exe/env
  `$ENVOY_DOCKER_BUILD_DIR` points).
  To leverage a [bazel remote cache](https://github.com/envoyproxy/envoy/tree/master/bazel#advanced-caching-setup) add the http_remote_cache endpoint to
  -the BAZEL_BUILD_OPTIONS environment variable
  +the BAZEL_BUILD_EXTRA_OPTIONS environment variable
  ```bash
  -BAZEL_BUILD_OPTIONS='--remote_http_cache=http://127.0.0.1:28080' ./ci/run_envoy_docker.sh './ci/do_ci.sh bazel.release'
  +./ci/run_envoy_docker.sh "BAZEL_BUILD_EXTRA_OPTIONS='--remote_http_cache=http://127.0.0.1:28080' ./ci/do_ci.sh bazel.release"
  ```
  The `./ci/run_envoy_docker.sh './ci/do_ci.sh <TARGET>'` targets are:
  