This change will help users avoid unnecessary procedure to
complete `bundle install` outside the container.
The explanation
> To generate the PAPERCLIP_SECRET, SECRET_KEY_BASE, and
> OTP_SECRET, you may use:
>
> ```
> rake secret
> ```
gives users a false impression that secrets have to be filled in
before the image can be built. Here, with introduction of CLD3,
completion of `bundle install` became more involved procedure to
prepare tools and libraries as well as higher version of Ruby.
58d6dcb46a