Speed up local builds of the admin container

admin/Dockerfile

@@ -3,12 +3,7 @@ FROM python:3
 RUN mkdir -p /app
 WORKDIR /app
 
-COPY mailu ./mailu
-COPY migrations ./migrations
-COPY manage.py .
 COPY requirements.txt .
-COPY start.sh /start.sh
-
 RUN pip install -r requirements.txt
 
 # Temporarily install certbot from source while waiting for 0.10
@@ -17,6 +12,11 @@ RUN git clone https://github.com/certbot/certbot /certbot \
  && pip install -e ./acme \
  && pip install -e ./
 
+COPY mailu ./mailu
+COPY migrations ./migrations
+COPY manage.py .
+COPY start.sh /start.sh
+
 RUN pybabel compile -d mailu/translations
 
 CMD ["/start.sh"]