Welcome, Mark! 
Rebuilding the app re-download Discourse, and will overwrite any change you made inside the container.
If you want to bring modifications to Discourse, editing these files this way is not the proper way to do so.
Customize features of Discourse is usually made either with a theme component:
Or a plugin:
If you have some coding knowledge, I suggest you have a look at these links. 
This link might be particularly interesting:
At first glance, it seems to do exactly what you’re looking for. It just targets another file in the same parent folder.