优化 Dockerfile，移除用户创建和不必要的文件复制；更新 start_gradio.sh 权限

Dockerfile

@@ -1,15 +1,11 @@
 FROM stepfun2025/vllm:step-audio-2-v20250909
 
-RUN useradd -m -u 1000 user
-
 WORKDIR /app
 
 COPY --chown=user ./requirements.txt requirements.txt
 
 RUN pip install --no-cache-dir --upgrade -r requirements.txt
 
-COPY --chown=user . /app
-
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt