Add support for updating global ARGs

[MartinBasti]

ARGs defined before first FROM in dockerfile are considered to be global ARGs and can be inherited into stages.

https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact

An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM. To use the default value of an ARG declared before the first FROM use an ARG instruction without a value inside of a build stage

Add ability to list and update global ARGs