fix live-reloading in docker (#2022)

Fix cotes2020#2021 : The webpage hosted by docker would not be automatically
reloaded because the port used for live-reloading (port 35729) is not
forwarded.

Author: wayne-tsai

docker-compose.yml

@@ -6,5 +6,6 @@ services:
     build: .
     ports:
       - 8080:8080
+      - 35729:35729
     volumes:
       - .:/srv/jekyll