Initialize build args from stage base

[daghack]

Currently, when we initialize a new dispatchState, buildArgs are not carried forward from the base.
This causes a subtle issue while linting where a Dockerfile such as

FROM alpine AS declared
ARG foo
COPY $foo .

FROM declared AS dependent
COPY $foo .

Will erroneously emit a UndeclaredVariable linting warning for the occurrence of $foo in the dependent stage.

This updates dispatchState to initialize buildArgs from its base.