Traefik reverse proxy to external site

api_flutter.yaml

http:
  middlewares:
    api-flutter-header:
      headers:
        customRequestHeaders:
          Host: "master-api.flutter.dev"
  
  routers:
    api-flutter-router:
      service: api-flutter-service
      rule: "Host(`api.flutter.domain.tld`)"
      tls:
        certResolver: le
      middlewares: 
        - "api-flutter-header"

  services:
    api-flutter-service:
      loadBalancer:
        servers:
          - url: "https://master-api.flutter.dev"

docker-compose.yaml

version: "3.3"
services:
  # Traefik is the reverse proxy
  traefik:
    container_name: traefik
    image: "traefik:v2.7.0-rc2"
    restart: unless-stopped
    ports:
      - 80:80
      - 443:443
    volumes:
      - /etc/localtime:/etc/localtime:ro
      # Add Docker as a mounted volume, so that Traefik can read the labels of other services
      - /var/run/docker.sock:/var/run/docker.sock:ro
      # Logs in root directory
      - /root/traefik/log:/log
      # Mount the volume to store the certificates
      - /root/traefik/certificates:/certificates
      # Load balancer to api.flutter.dev
      - /root/traefik/api_flutter.yaml:/api_flutter.yaml:ro
    command:
      # Create an entrypoint "http" listening on port 80
      - --entrypoints.http.address=:80
      # Create an entrypoint "https" listening on port 443
      - --entrypoints.https.address=:443
      # Logs
      - --accesslog
      - --log
      - --log.level=WARN
      - --log.format=json
      - --log.filePath=/log/log.json
      - --certificatesresolvers.le.acme.httpchallenge=true
      - --certificatesresolvers.le.acme.email=email@domain.tld
      - --certificatesresolvers.le.acme.storage=/certificates/acme.json
      - --certificatesresolvers.le.acme.httpchallenge.entrypoint=http

      # Redirect to api.flutter.dev
      - --providers.file.filename=/api_flutter.yaml