Fix missing -y on the GC-Sdk install

Change-Id: I2c53cbe97488391185dc5f7a6b2726e0df78c935
diff --git a/jenkins-docker/slave-release/Dockerfile b/jenkins-docker/slave-release/Dockerfile
index 04675a8..e90b510 100644
--- a/jenkins-docker/slave-release/Dockerfile
+++ b/jenkins-docker/slave-release/Dockerfile
@@ -5,7 +5,7 @@
     export CLOUD_SDK_REPO="cloud-sdk-$(lsb_release -c -s)" && \
     echo "deb http://packages.cloud.google.com/apt $CLOUD_SDK_REPO main" > /etc/apt/sources.list.d/google-cloud-sdk.list && \
     curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add - && \
-    apt-get update && apt-get install google-cloud-sdk && \
+    apt-get update && apt-get install -y google-cloud-sdk && \
     apt-get clean
 
 COPY gerrit-release.sh /usr/local/bin