dockerfile: improve error messages and add suggest to flag parsing

[tonistiigi]

Improve the error messages for error messages if FROM flags if they are caused by mismatched variables.

Additional commits improve the messages for UndeclaredArgInFrom and UndefinedVar warnings.

Example:

Dockerfile:1
--------------------
   1 | >>> FROM --platform=$BULIDPLATFORM alpine
   2 |     RUN apk add git
   3 |
--------------------
ERROR: failed to solve: failed to parse platform : "" is an invalid component of "": platform specifier component must match "^[A-Za-z0-9_-]+$": invalid argument

Dockerfile:1
--------------------
   1 | >>> FROM --platform=$BULIDPLATFORM alpine
   2 |     RUN apk add git
   3 |     
--------------------
ERROR: failed to solve: empty platform value from expression $BULIDPLATFORM (did you mean BUILDPLATFORM?)

[crazy-max]

Nice will be much easier to grasp for people!