Bump Docker image version

Closes #26 (closed)

Long-overdue version bump, for the docker Docker image, from 20-10-2 to 20-10-21.

Impacts:

• downloaded docker images in CI.
• installation of docker client.

Manual test

docker pull docker:20.10.21-git
docker pull docker:20.10.21-dind
curl "https://download.docker.com/linux/static/stable/x86_64/docker-20.10.21.tgz" -o "/tmp/docker.tar.gz"