At the moment, the "ensure-docker" role is broken due to the fact
that we're not mirroring the Docker upstream repositories for that
architecture.
This patch adds "arm64" to the list of architectures for Focal, but
it purposely ignored Xenial and Bionic as those are older distros
that are likely not using arm64 (or they would have noticed it was
broken!)
Change-Id: I33d62dc13fa786c15b352c012f9798348d09b8b0
748090b7ab