support .env files

[ndeloof]

initially reported as https://github.com/docker/ecs-plugin/issues/266

docker-compose do support .env files for variable interpolation, compose-cli should replicate this behaviour
see https://github.com/compose-spec/compose-go/blob/31db906dcfbdf0d87a21e69ffaadff485af140c8/cli/options.go#L103

[mattyweb]

seconded. the practical implication here is that we need to have separate compose files for each environment. our preference has been to share the compose file and tailor a .env file for staging, CI, production, etc.

it's not clear from the docs that this doesn't work or what the alternatives are; e.g. can environment variables be set from the shell?

[kaka-ruto]

I am using a .env file and get the following error on running docker compose up

service web doesn't define a Docker image to run: incompatible attribute

This is what I have

# .env
IMAGE_REPOSITORY='12345.region.amazonaws.com/repo-name'

# docker-compose.yml
version: "3.9"

services:
  web:
    image: "${IMAGE_REPOSITORY}"