To build a simple flask cluster with nginx(lbs).

docker_flasks_lbs_with_nginx.yml

version: '3.7'
services:
  app1:
      container_name: app1-container
      hostname: app1
      restart: always
      build:
        context: ./app
        dockerfile: Dockerfile
        args:
          - PYTHON_VERSION=${PYTHON_VERSION}
          - AUTHOR=${AUTHOR}
          - CONTAINER_LOGFILE_PATH=${CONTAINER_LOGFILE_PATH}
      volumes:
        - socket_app1:${SOCKET_PATH} # Share the socket file for Nginx.
        - ${APP_CODE_PATH_HOST}:${APP_CODE_PATH_CONTAINER}
        - ${LOCAL_LOGFILE_PATH}:${CONTAINER_LOGFILE_PATH}
      environment:
        - NODE_NAME=${NODE_NAME1}
        - CONTAINER_LOGFILE_PATH=${CONTAINER_LOGFILE_PATH}
        - APP_CODE_PATH_CONTAINER=${APP_CODE_PATH_CONTAINER}
        - SOCKET_FILE=${SOCKET_FILE}
        - SOCKET_PATH=${SOCKET_PATH}
        - USER_ID=${USER_ID}
        - GROUP_ID=${GROUP_ID}
        - PY-AUTORELOAD=${PY-AUTORELOAD}
  app2:
      container_name: app2-container
      hostname: app2
      restart: always
      build:
        context: ./app
        dockerfile: Dockerfile
        args:
          - PYTHON_VERSION=${PYTHON_VERSION}
          - AUTHOR=${AUTHOR}
          - CONTAINER_LOGFILE_PATH=${CONTAINER_LOGFILE_PATH}
      volumes:
        - socket_app2:${SOCKET_PATH} # Share the socket file for Nginx.
        - ${APP_CODE_PATH_HOST}:${APP_CODE_PATH_CONTAINER}
        - ${LOCAL_LOGFILE_PATH}:${CONTAINER_LOGFILE_PATH}
      environment:
        - NODE_NAME=${NODE_NAME2}
        - CONTAINER_LOGFILE_PATH=${CONTAINER_LOGFILE_PATH}
        - APP_CODE_PATH_CONTAINER=${APP_CODE_PATH_CONTAINER}
        - SOCKET_FILE=${SOCKET_FILE}
        - SOCKET_PATH=${SOCKET_PATH}
        - USER_ID=${USER_ID}
        - GROUP_ID=${GROUP_ID}
        - PY-AUTORELOAD=${PY-AUTORELOAD}

  loadbalancer:
      container_name: loadbalancer-container
      build:
        context: ./loadbalancer
        dockerfile: Dockerfile
        args:
          - AUTHOR=${AUTHOR}
          - NGINX_VERSION=${NGINX_VERSION}
      depends_on:
          - app1
          - app2
      volumes:
          # Load the socket file from volumes.
          - socket_app1:/socket_app1
          - socket_app2:/socket_app2
          - ./server_logs:/var/log/nginx
      ports:
          - '18088:80'
      environment:
          - NGINX_UPSTREAM_CONTAINER=${NGINX_UPSTREAM_CONTAINER}
          - SOCKET_FILE=${SOCKET_FILE}

volumes:
  socket_app1:
  socket_app2: