The docker-compose file mounts a volume called /app which is where the server will be sourced from.
There are also cache volumes mounted for the PostGRES data directory and the bundle install directory, so that you don't have to wait for bundles to install every compose up. To bust the cache, simply run docker-compose down -v to destroy the volumes, and then docker-compose up as usual.
The docker-start.sh file runs a yarn install, bundle check || bundle install to check if any gems are missing and if so, install them. Kills any artifacts blocking the server from running if the container is shutdown forcibly, and binds the server to port 3000 along with some self-signed SSL certificates, to comply with the new Chrome forced HTTPS protocol. This fixes the ERR_SSL_PROTOCOL white screen in Chrome when trying to use rails server or rails s, and the Puma Error in the rails logs: HTTP parse error, malformed request (): #<Puma::HttpParserError: Invalid HTTP format, parsing fails.
Simply drop these files into your application folder, change the docker-compose file to map your ports to fit your needs, and adapt the docker-start.sh to match the port mappings in your docker-compose file.