Fixing hot reloading for Gatsby site

Exposing ports 8000 and 9000 for gatsby develop and gatsby serve respectively
Set CHOKIDAR_USEPOLLING to allow clients to poll for changes (used in hot reload)
Set INTERNAL_STATUS_PORT for websockets to listen to for Gatsby's Hot Module Reload technology
Update the README to refer to --host 0.0.0.0 so that the client can talk to the container

ardalis / DevIQ-gatsby