Currently imagec is only able to pull images by tag. For example:

docker pull busybox:1.24.2

We need to provide support for pulling images via image digest:

docker pull busybox@sha256:4a731fb46adc5cefe3ae374a8b6020fc1b6ad667a279647766e9a3cd89f6fa92

While vic (via imagec) will attempt a pull when a user provides a digest, it's not pulling via the digest -- it's actually pulling via the latest tag. Since the digest is ignored the wrong image might be pulled.

Depends on #1484