Allow configuration of the agent's statsd server port through the
statsd_port option.

Configure the Puma plugin using env vars. It cannot access the normal
AppSignal config, because the gem isn't loaded in the main process.

Requires an agent update from
appsignal/appsignal-agent#995

- Rely on APPSIGNAL_RUNNING_IN_CONTAINER config option value before
  other environment factors to determine if the app is running in a
  container.
- Fix container detection for hosts running Docker itself.
- Add APPSIGNAL_STATSD_PORT config option.