The trick to read env in docker image

arg_map.md

it is possible to map the .env environment variables to ARGS to be used by the Dockerfile during build.

docker-compose.yml

version: "x"
services:

  xxx:
    build:
    # there is the space between context and .
      context: .
      dockerfile: ./Dockerfile
      args:
        - PORT=${PORT}
    volumes:
       ...

Dockerfile

FROM xx
ARG PORT

ENV PORT "$PORT"

EXPOSE ${PORT}
...

In the root folder, you can define the ARG in .env.