docker: add podman as container runtime for Buster

roles/base/tasks/apt.yml

roles/docker/defaults/main.yml

roles/docker/files/registries.conf

roles/docker/tasks/buster_upgrade.yml

roles/docker/tasks/docker.yml

roles/docker/tasks/main.yml

roles/docker/tasks/podman.yml

roles/docker/tasks/start.yml

roles/docker/templates/run.sh.j2

roles/docker/templates/systemd.j2

test/integration-test-docker.yml