Good luck!

Ok! Let's do this.

Step 1: You need to point your domain name at your server! If you haven't already, add an A record with your domain registrar that resolves to the IP address of your GCE instance. We'll need this in order to set up Caddy.

Step 2: Make a new file named Caddyfile. This is the contents of the configuration for the caddy server. It should look like this:

yourdomainname.com {
  header / {
    Strict-Transport-Security "max-age=31536000; includeSubdomains; preload"
    Content-Security-Ploicy "default-src 'none';base-uri 'none';manifest-src 'self';script-src 'self' 'unsafe-eval';style-src 'self' 'unsafe-inline';img-src 'self' data: blob:;font-src 'self';connect-src 'self';media-src 'self';frame-src 'self';child-src 'self'"
    X-Robots-Tag "none" # This will ask search engines _not_ to index your site! If you want to be indexed, remove this
  }
  proxy / localhost:8080 {
    transparent
    insecure_skip_verify
    websocket
  }
}

Happy to answer any questions about what's happening above!

Step 3: We need to make some changes to the docker-compose.yml again. Here's what we need:

version: '2'

volumes:
  nextcloud:
  db:
  caddy_data:

services:
  db:
    image: mariadb
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    restart: always
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=
      - MYSQL_PASSWORD=
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud

  app:
    image: nextcloud
    ports:
      - 8080:80  # Note that this is a back to 8080 again!
    links:
      - db
    volumes:
      - nextcloud:/var/www/html
    restart: always

  caddy:
    image: caddy
    volumes:
      - "path/to/Caddyfile:/etc/caddy/Caddyfile" # Make sure you replace "path/to/Caddyfile" with the actual path to your Caddyfile that you made earlier!
      - "caddy_data:/data"
    ports:
      - 80:80
      - 443:443
    restart: always

Then if you restart docker-compose (sudo docker-compose down && sudo docker-compose up -d), you should find yourself able to get back to that settings page by going to your domain name in your browser!

Full disclosure: I've never run Caddy via docker-compose before, so it's possible I mucked up that config file. Let me know if you run into any trouble!