grpc: Unavailable response when gRPC server run in Docker

My code (written in Go, which just creates a Bucket via gRPC) succeeds when the `storage_testbench` server is run directly in a `virtualenv`, but fails when the server is running inside of a Docker container. The error reads `rpc error: code = Unavailable desc = connection closed before server preface received`.

The container was started with the following command:
`docker run --name storage_testbench --rm -d -p 9000:9000 -p 8888:8888 gcr.io/cloud-devrel-public-resources/storage-testbench`

The gRPC server was started with the following command:
`curl -s "http://localhost:9000/start_grpc?port=8888"`