Previously `WithPlugins` only supported passing image URIs "by tag" --
e.g. `gcr.io/my-repo/my-image:v0.1.2`. With this commit, we add support
for pulling "by digest" -- e.g.
`gcr.io/my-repo/my-image@sha256:a75f9e8c3ced3943515f249597be389f8233e1258d289b11184796edceaa7dab`
Signed-off-by: Eric Fried <efried@redhat.com>