FROM ARGS are not checked

[mxbossard]

• This is a bug report
• This is a feature request
• I searched existing issues before opening this one

Expected behavior

With supplied Dockerfile :
ARG foo bar mean nothing, there is no equal sign. The linter should complain.
FROM is referencing a missing ${baz} ARG. The linter should complain.
ARG pif paf mean nothing, there is no equal sign. The linter should complain.

Actual behavior

The linter do not complain.

Steps to reproduce the behavior

Dockerfile (if relevant)

ARG foo bar
FROM alpine${foo}:${baz}
ARG pif paf

[mxbossard]

I tested ARG foo bar on Docker and it works like if we declared 2 args foo and bar on 2 separate lines.
But this behavior is not documented, so should the linter support it ?