fix: set build context to app/ in prod compose

The compose file lives in /srv/sum/nohype/ while the code is in
/srv/sum/nohype/app/ so the Dockerfile is at app/Dockerfile.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker-compose.prod.yml

@@ -1,6 +1,6 @@
 services:
   web:
-    build: .
+    build: app
     working_dir: /app
     command: >
       sh -c "python manage.py tailwind install --no-input &&