Add demo-* CI builds

Similar to how we do it google-cloud-cpp, add builds for the more common OS distributions we support. The docker files exist, but need to be updated and the triggers installed in gcb.

Compare the existing CI approach in google-cloud-cpp, then inspect the repository's Docker files and current Google Cloud Build configuration. Identify the common supported OS distributions, update the Docker files, and install the corresponding GCB triggers; done means the demo-* builds run successfully for those distributions.

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