Invalid digests in a private registry with ports should be flagged as an error

[rcjsuen]

FROM localhost/node@
FROM localhost:1234/node@
FROM 123.22.33.123/user/image@
FROM 123.22.33.123:2345/user/image@

These FROM instructions are clearly invalid but we don't seem to be considering all of them. Only the first and third lines are being flagged as an error.

$ docker run -v `pwd`/Dockerfile:/Dockerfile rcjsuen/dockerfile-utils lint /Dockerfile

Unable to find image 'rcjsuen/dockerfile-utils:latest' locally
latest: Pulling from rcjsuen/dockerfile-utils
ff3a5c916c92: Pull complete
4d34b95ee50c: Pull complete
58006ba8d4ce: Pull complete
115c0912b672: Pull complete
23475bf850aa: Pull complete
7c7babd76ce1: Pull complete
ac1d2627637c: Pull complete
Digest: sha256:fe13d08564b5770f0867d95e2027dd730e72f78f86c4de63f4ceb77ed5b93985
Status: Downloaded newer image for rcjsuen/dockerfile-utils:latest
Line: 1
FROM localhost/node@
     ^^^^^^^^^^^^^^^
Error: invalid reference format

Line: 3
FROM 123.22.33.123/user/image@
     ^^^^^^^^^^^^^^^^^^^^^^^^^
Error: invalid reference format