Bump debian from buster-20201117 to buster-20201209 in /docker/cluster-test#1

dependabot[bot] wants to merge 1 commit intomasterJoshLind/diem:masterfrom

dependabot/docker/docker/cluster-test/debian-buster-20201209JoshLind/diem:dependabot/docker/docker/cluster-test/debian-buster-20201209Copy head branch name to clipboard