Fix backend Dockerfile paths for root context

backend/Dockerfile

@@ -5,10 +5,10 @@ WORKDIR /app
 ENV PYTHONDONTWRITEBYTECODE=1 \
     PYTHONUNBUFFERED=1
 
-COPY requirements.txt .
+COPY backend/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
-COPY app ./app
+COPY backend/app ./app
 COPY data/branding /app/data/branding
 
 EXPOSE 8000