Simplify deployment on Google Cloud Run / Add simple gRPC health check

[jgavris]

• Resolves https://github.com/znly/bazel-cache/issues/5, at least, made it simpler to configure.
• Have been running in production for a few weeks with no issues so far.
• Removes .git from the docker image for an even smaller image.
• Uses docker buildx to simplify cross-compilation of the image.
• Defaults to serve command.
• Combines --port and --port_from_env into a single Viper injected / defaulted env var.
• Happy to clean this up, I had staged a gitignored deploy.sh file.

For this image, I recommend 8GiB memory, 4 vCPU, 10 requests per container, and second generation execution environment. For autoscaling, maybe 6-8 minimum and maximum depends on how bursty your traffic is. Monitor the cloud run metrics for the service for a few days.