Update gcloud to 389.0.0 (GoogleCloudPlatform#8082)

The existing version lacks `gcloud artifacts` which is blocking some tests.
This PR upgrades gcloud to 389.0.0.

Co-authored-by: Dan Lee <71398022+dandhlee@users.noreply.github.com>

.kokoro/docker/Dockerfile

@@ -165,7 +165,7 @@ RUN rm /tmp/get-pip.py
 RUN pip install --no-cache-dir virtualenv
 
 # Setup Cloud SDK
-ENV CLOUD_SDK_VERSION 360.0.0
+ENV CLOUD_SDK_VERSION 389.0.0
 # Use system python for cloud sdk.
 ENV CLOUDSDK_PYTHON python3.6
 RUN wget https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-sdk-$CLOUD_SDK_VERSION-linux-x86_64.tar.gz