Remount /makigas/tmp using tmpfs

Docker doesn't seem to properly clean-up the Rails application when it
stops, so the remaining server.pid confuses Rails the very next time it
starts up.

docker-compose.yml

@@ -19,6 +19,7 @@ services:
       - database
     volumes:
     - '.:/makigas'
+    tmpfs: /makigas/tmp
     ports:
       - '3000:3000'
     environment: