parse arg commands at the top of dockerfiles (#404)

* parse arg commands at the top of dockerfiles

* fix pointer reference bug and remove debugging

* fixing tests

* account for meta args with no value

* don't take fs snapshot if / is the only changed path

* move metaArgs inside KanikoStage

* removing unused property

* check for any directory instead of just /

* remove unnecessary check

integration/dockerfiles/Dockerfile_test_meta_arg

@@ -0,0 +1,17 @@
+ARG REGISTRY=gcr.io
+ARG REPO=google-appengine
+ARG WORD=hello
+ARG W0RD2=hey
+
+FROM ${REGISTRY}/${REPO}/debian9 as stage1
+
+# Should evaluate WORD and create /tmp/hello
+ARG WORD
+RUN touch /${WORD}
+
+FROM ${REGISTRY}/${REPO}/debian9
+
+COPY --from=stage1 /hello /tmp
+
+# /tmp/hey should not get created without the ARG statement
+RUN touch /tmp/${WORD2}