Upgrade CI to Docker 19.03.9

Closes spring-projectsgh-21525

Author: mbhave

ci/images/get-docker-url.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 set -e
 
-version="19.03.8"
+version="19.03.9"
 echo "https://download.docker.com/linux/static/stable/x86_64/docker-$version.tgz";