Add a variety of files for [gitpod.io][] support, including the
platform-specific `.gitpod.yml` file, and the `.gitpod.Dockerfile`.

Outside of those files, make `gitpod.io`-specific overrides to
`config/environments/development.rb` to support serving the application
from a `gitpod.io`-controller subdomain.

[gitpod.io]: https://gitpod.io