build(avatar): preserve backend dependency cache

digital-avatar-app/backend/Dockerfile

@@ -1,12 +1,5 @@
 FROM python:3.10-slim
 
-ARG APP_GIT_SHA=unknown
-ARG APP_BUILD_TIME=unknown
-ENV APP_GIT_SHA=${APP_GIT_SHA} \
-    APP_BUILD_TIME=${APP_BUILD_TIME}
-LABEL org.opencontainers.image.revision=${APP_GIT_SHA} \
-      org.opencontainers.image.created=${APP_BUILD_TIME}
-
 WORKDIR /app
 
 # 后端依赖（fastapi/uvicorn/sqlalchemy/pypdf/python-docx/openpyxl 等）均为纯 Python wheel，
@@ -14,6 +7,13 @@ WORKDIR /app
 COPY requirements.txt .
 RUN pip install --no-cache-dir --timeout 120 --retries 10 -i https://pypi.tuna.tsinghua.edu.cn/simple -r requirements.txt
 
+ARG APP_GIT_SHA=unknown
+ARG APP_BUILD_TIME=unknown
+ENV APP_GIT_SHA=${APP_GIT_SHA} \
+    APP_BUILD_TIME=${APP_BUILD_TIME}
+LABEL org.opencontainers.image.revision=${APP_GIT_SHA} \
+      org.opencontainers.image.created=${APP_BUILD_TIME}
+
 COPY . .
 
 # 后端使用 SQLite（avatar.db 落在 /app 内），平铺结构以 `uvicorn main:app` 启动