refactor: fix handling of string andd array CMD and ENTRYPOINT values

Previous versions extracted the values as strings, prefixing the values with `/bin/sh -c '$CMD'` unnecessarily. This caused issues where we would incorrectly overrride these values when an app had a predeploy script.

Closes #3143

tests/apps/dockerfile-entrypoint/Dockerfile

@@ -0,0 +1,10 @@
+FROM ruby:2.5.1
+
+RUN mkdir /app
+WORKDIR /app
+
+ADD Procfile Procfile
+ADD entrypoint entrypoint
+ADD app.json app.json
+
+ENTRYPOINT ["./entrypoint"]