Don't clear ARGS on FROM

[adamgordonbell]

The following behavior is causing me some issues:

test-stuff:
    ARG BLA=name
    FROM alpine
    ENV BLA2=$BLA
    RUN env

I would expect BLA2 to be set, but it is not:

 +test-stuff | --> RUN env
         +test-stuff | SHLVL=2
         +test-stuff | HOME=/root
         +test-stuff | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
         +test-stuff | BLA2=
         +test-stuff | PWD=/