Provide a way to regenerate rails credentails

[zyc9012]

Currently there is a config/credentials.yml.enc in the project, new projects forked from this should replace it with a new one for safety, it will be nice to provide a way to regenerate the credentials.

Steps to regenerate manually:

1. delete credentials.yml.enc
2. go into docker container, run rails credentials:edit
3. cat config/master.key, put the key as docker env RAILS_MASTER_KEY, and delete config/master.key