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/app.json

@@ -0,0 +1,7 @@
+{
+  "scripts": {
+    "dokku": {
+      "predeploy": "touch /app/predeploy.test"
+    }
+  }
+}