OTP application callback — only relevant when this project is compiled
and released as its own standalone deployment (the Docker image). When
pusher_server is instead pulled in as a hex dependency of another
app, this starts an empty supervisor and does nothing: the host app is
expected to add {PusherServer, opts} to its own supervision tree
explicitly (see PusherServer's moduledoc).
The distinction is made via config :pusher_server, :standalone?,
which is set to true only in this project's own config/runtime.exs.
A dependency's config/runtime.exs is not evaluated when building
someone else's release, so this stays false (the safe default) for
anyone who adds this as a dependency.