Bump up python version of Dockerfile (#3042)

docker/Dockerfile

@@ -2,7 +2,7 @@
 # https://nvflare.readthedocs.io/en/main/quickstart.html#containerized-deployment-with-docker
 # This Dockerfile is primarily for building Docker images to publish for dashboard.
 
-FROM python:3.8
+FROM python:3.9
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get -y install zip
 COPY nvflare /opt/NVFlare/nvflare
 COPY setup.py /opt/NVFlare/setup.py