Attempt to fix current Superset behavior not loading anything

Our attempts to upgrade Superset-next to 3.1.1 and 3.1.2 have all yielded the same results: we get past the CAS login, and then.. nothing. We don't see any errors in the browser console, nginx or superset logs.

I have a hunch that this change might have broken more things than intended. The initial intention was to remove statics from the backend image, as they were served by nginx.

This is pretty much a hail Mary to see whether restoring that behavior "fixes" Superset.

Bug: T358674