Update dockerfile to use UV

Change backend context from . to ./backend in docker-compose.
This is necessary for copying the pyproject.toml and uv.lock files from project root level

docker-compose.yml

@@ -1,7 +1,8 @@
 services:
   backend:
     build:
-      context: ./backend
+      context: .
+      dockerfile: ./backend/Dockerfile
     container_name: shadowbroker-backend
     ports:
       - "8000:8000"