Fix #519 capital letter in stage names

update stage code so that when comparing the BaseName of
a stage against the recorded, lowercase version of a Stage name
the BaseName is also lowercased.

integration/dockerfiles/Dockerfile_test_from_multistage_capital

@@ -0,0 +1,13 @@
+FROM alpine as base_stage
+
+RUN echo base_stage
+
+
+FROM base_stage as BUG_stage
+
+RUN echo BUG_stage
+
+
+FROM BUG_stage as final_stage
+
+RUN echo final_stage