Update python Docker tag to v3.8 (#4444)

Co-authored-by: Charles Engelke <engelke@google.com>

memorystore/redis/cloud_run_deployment/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