Warn if base image's tag is invalid

[rcjsuen]

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

FROM alpine:

$ docker build .
Sending build context to Docker daemon   1.88MB
Step 1/1 : FROM alpine:
invalid reference format

FROM alpine:^

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

FROM alpine:a66^

$ docker build .
Sending build context to Docker daemon  1.881MB
Step 1/1 : FROM alpine:a66^
invalid reference format