chore(deps): update python docker tag to v3.8 (GoogleCloudPlatform#4583)

Author: renovate-bot

trace/cloud-trace-demo-app-opentelemetry/app/Dockerfile

@@ -1,6 +1,6 @@
 # Use the official lightweight Python image.
 # https://hub.docker.com/_/python
-FROM python:3.7-slim
+FROM python:3.8-slim
 
 # Copy local code to the container image.
 ENV APP_HOME /app