modify mysql collation

packages/slackbot-proxy/docker-compose.dev.yml

@@ -11,6 +11,7 @@ services:
       - MYSQL_USER=growi-slackbot-proxy
       - MYSQL_PASSWORD=YrkUi7rCW46Z2N6EXSFUBwaQTUR8biCU
       - MYSQL_DATABASE=growi-slackbot-proxy
+    command: mysqld --character-set-server=utf8mb4 --collation-server=utf8mb4_general_ci
     volumes:
       - /data/db