optimize Dockerfile caching

Dockerfile

@@ -6,7 +6,9 @@ ENV HOMESERVER_URL=https://matrix.org \
     ADMIN_ROOM=!jaeisofjaosiefjoi:matrix.org
 
 WORKDIR /app
-COPY requirements.txt scrape.py ./
+
+COPY requirements.txt ./
 RUN pip install -r requirements.txt
 
+COPY scrape.py ./
 CMD [ "python", "scrape.py" ]