Use `rails` as the entrypoint

[durierem]

Suggestion to add rails as the container ENTRYPOINT which enables a more streamlined experience by not requiring the user to type in rails after docked.

I'm wondering if this could be pushed further by renaming the alias something like drails (docked-rails) to highlight the fact that it's running Rails. One could go even further and naming the alias rails but I think it would cause issues and confusion with potential native rails installations.