Add /public/uploads for file upload gem Shrine (#3333)

When using a file upload gem called Shrine, its file system mode put images under `public/uploads`.

https://github.com/shrinerb/shrine/blob/053bcf297e092e9695731fb9b67a86780e898203/doc/storage/file_system.md

Rails.gitignore

@@ -66,3 +66,4 @@ yarn-debug.log*
 # Ignore uploaded files in development
 /storage/*
 !/storage/.keep
+/public/uploads