dockerTools.pullImage: Skopeo pulls images by digest

Skopeo is used to pull images from a Docker registry (instead of a
Docker deamon in a VM).

An image reference is specified with its name and its digest which is
an immutable image identifier (unlike image name and tag).

Skopeo can be used to get the digest of an image, for instance:
$ skopeo inspect docker://docker.io/nixos/nix:1.11 | jq -r '.Digest'