docker: pre-download transformer weights

Dockerfile

@@ -28,6 +28,7 @@ WORKDIR /app/backend
 
 COPY ./backend/requirements.txt ./requirements.txt
 RUN pip3 install -r requirements.txt
+RUN python -c "from sentence_transformers import SentenceTransformer; model = SentenceTransformer('all-MiniLM-L6-v2')"
 
 COPY ./backend .