Add a Dockerfile to build the builder

Our Gradle build now requires three programs to build: Java, npm and gcloud. There are no existing images that contain all of them. Even if there were, they probably come from some random Joe on the Internet and we cannot trust the image to be free of malwares. Therefore we need to build our own builder.

The builder images will be built by Cloud Build and upload to our container registry. We should periodically rebuild it to pull in the latest security updates both for the base Ubuntu image, and for the components that we install. I have not figured out a way to do that yet. For now we'll just trigger Cloud Build manually once in a while.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=234009343

cloudbuild-nomulus.yaml

@@ -23,7 +23,7 @@ steps:
 - name: 'alpine'
   args: ['sh', '-c', 'cp -r nomulus-internal/* .']
 # Build the deployment files.
-- name: 'google/cloud-sdk'
+- name: 'gcr.io/${PROJECT_ID}/builder'
   args:
     - './gradlew'
     - 'stage'