How to get port of docker-compose from env_file?

The env_file optin will only set environment variables in the docker container itself. Not on the host which is used during the compose ‘build’.

To define your port as env var you should use the .env file as described here

In your case create a .env file which contains::

PORT=5000

and the docker-compose.yml:

version: '3'
services:
  flask:
    build:
      context: ./flask
      dockerfile: Dockerfile_flask
    ports:
     #- "5000:5000"
     - "${PORT}:${PORT}"  # I want to set port defined in the env file
    volumes:
      - ./logs:/app/flask/log
    restart: always

If you want to add environment variable to your container using a env_file you can add it again.

To make it fully clear this example:
A postgres started in compose. The environment variables in the my-env-file are known inside the container, the env var inside .env is used during the docker-compose up process.

a .env file with:

PORT=5432

a my-env-file with:

POSTGRES_USER=dev
POSTGRES_PASSWORD=secret
POSTGRES_DB=db

and the docker-compose.yml:

version: ‘3.3’
services:
  postgres:
   image: postgres:9.6
   container_name: postgres
   env_file:
     - my-env-file
   ports:
     - ${PORT}:${PORT}