Using variables in FROM confuses the validator

[rcjsuen]

Using variables is valid in the base image argument of a FROM but it seems to validator won't ignore it properly.

ARG image=scratch
FROM ${image:-alpine}

$ docker build .
Sending build context to Docker daemon  1.774MB
Step 1/2 : ARG image=scratch
Step 2/2 : FROM ${image:-alpine}
 --->
No image was generated. Is your Dockerfile empty?

$ dockerfile-utils lint Dockerfile
Line: 2
FROM ${image:-alpine}
             ^^^^^^^^
Error: invalid reference format