Dockerfile: Specify fully qualified image name

[akosiaris]

Why: podman and containerd require fully qualified names for Docker images, meaning that the hostname of the registry needs to be present. Docker defaults to docker.io when that is missing, but both podman and containerd require specific configuration for that to happen on the part of the operator.

What: Make life easier for users of alternate container engines by specifying the fully qualified names for images in the Dockerfile