Docker install assumes `/etc/docker` exists

[BretFisher]

Docker v23+ doesn't create the /etc/docker/ directory by default, so we need a sudo mkdir -p /etc/docker to prevent the docker install from failing/repeating.

PR incoming.

container.training/prepare-vms/lib/commands.sh

Lines 269 to 272 in 1aa0e06

	if ! [ -f /etc/docker/daemon.json ]; then
	echo '{\"registry-mirrors\": [\"https://mirror.gcr.io\"]}' | sudo tee /etc/docker/daemon.json
	sudo systemctl restart docker
	fi