Use gradlew in Cloud Build

Also moved pushing tags to the private repo to the last step as we do not want
to tag the configs unless the build is successful.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=223982622

.gcloudignore

@@ -4,4 +4,3 @@ bazel-*/**
 gradle/**/build/**
 gradle/**/WEB-INF/**
 gradle/.*/**
-gradle/gradle/**