More details on the secret-key-base

[Marc-henryJ]

It seems that if we don't specify a SECRET_KEY_BASE it will be generated, but will also be re-generated after the cache or storage is deleted ?

Need to verify and add more info in the documentation about it :
https://doc.scalingo.com/languages/ruby/rails/start#specify-a-secret-key-base

https://github.com/Scalingo/ruby-buildpack/blob/9dc20d2905a4c615b98c2fe2c832f14bec1f1b48/lib/language_pack/rails41.rb#L21