Declaring default environment variables in docker-compose

Question

How to declare default environment variables under docker-compose?

Answer 1

Compose supports declaring default environment variables in an environment file named .env placed in the folder where the docker-compose command is executed (current working directory).

Example: The below example demonstrate how to declare default environmental variable for Docker Compose.

$ cat .env
TAG=v1.5

$ cat docker-compose.yml
version: '3'
services:
  web:
    image: "webapp:${TAG}"

When you run docker-compose up, the web service defined above uses the image webapp:v1.5. You can verify this with the `docker-compose config` command which prints your resolved application config to the terminal.

$ docker-compose config

version: '3'
services:
  web:
    image: 'webapp:v1.5'