use python 3.7, as python:3-slim is now using python 3.8. (#254)
use python 3.7, as python:3-slim is now using python 3.8. (#254)


GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo