Adds linter warning for using variables defined in parent stages

[daghack]

This adds a linter warning for the case where a build arg is declared, and then later used in a child stage without being redeclared in that stage. For example:

RUN echo "Building version"
ARG username=foo
RUN echo "declared username: $username"

FROM one AS two
# Causes the lint warning
RUN echo "out of scope arg username: $username, define ARG in current stage to use it"