Warn if base image's digest is invalid

[rcjsuen]

If the digest of a FROM is invalid, we should create a warning about it.

FROM alpine@sha25:x

$ docker build .
Sending build context to Docker daemon  1.881MB
Step 1/1 : FROM alpine@sha25:x
invalid reference format

[rcjsuen]

Leaving this issue open for creating a warning if the digest is invalid. See #20 for creating a warning a warning if the tag is invalid.