fix(gcb): Remove build steps from GCB (#5450)

We were building the testing image instead of the application image in
GCB, which means that an additional 5 minutes were spent on installing
test dependencies.

cloudbuild.yaml

@@ -9,6 +9,7 @@ steps:
  "--build-arg",
  "SOURCE_COMMIT=$COMMIT_SHA",
  "--destination=us.gcr.io/$PROJECT_ID/$REPO_NAME:$COMMIT_SHA",
+ "--target=application",
  "-f",
  "./Dockerfile",
  ]