Shadow elm-stuff build artifacts from local machine.

When build artifacts from local machine are reused in container, it
usually ends with "corrupt binary" error.

.dockerignore does not work for volumes so we define additional volume
to cover one path on existing mount:
https://stackoverflow.com/questions/29181032/add-a-volume-to-docker-but-exclude-a-sub-folder

Closes #653

docker-compose.yml

@@ -9,6 +9,7 @@ services:
     volumes:
       - .:/res
       - gems:/usr/local/bundle
+      - /res/ruby_event_store-browser/elm/elm-stuff/
     working_dir: /res
   mysql:
     image: mysql