chore: add volume define in Dockerfile to avoid data lose (#1718)

Add volume define in Dockerfile to protect data
Co-authored-by: Athurg Feng <athurg@gooth.org>

Dockerfile

@@ -34,5 +34,6 @@ EXPOSE 5230
 
 # Directory to store the data, which can be referenced as the mounting point.
 RUN mkdir -p /var/opt/memos
+VOLUME /var/opt/memos
 
 ENTRYPOINT ["./memos", "--mode", "prod", "--port", "5230"]