The Power of the .env File in Docker Compose

In this post, I’ll demonstrate how powerful and flexible the .env file can be when setting up a compose.yaml for Docker Compose. This approach allows for easy management of environment variables, making your Docker configurations more dynamic and manageable.

Let’s start with a simple .env file:

UBUNTU_VERSION=24.04

And a corresponding compose.yaml file:

services:
  test:
    image: ubuntu:${UBUNTU_VERSION}
    command: ["sh", "-c", "env"]

When you run the Docker Compose stack with the command docker compose up, you’ll see an output like this:

$ docker compose up
[+] Running 2/1
 ✔ Network tmp_default   Created                                                                          0.1s
 ✔ Container tmp-test-1  Created                                                                          0.1s
Attaching to test-1
test-1  | HOSTNAME=f9002b77bc79
test-1  | HOME=/root
test-1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
test-1  | PWD=/
test-1 exited with code 0

However, to make the variables defined in the .env file available within the Docker container, you need to add a couple of lines to your compose.yaml file:

services:
  test:
    image: ubuntu:${UBUNTU_VERSION}
    command: ["sh", "-c", "env"]
    env_file:
      - .env

After updating the compose.yaml file, run the docker compose up command again. This time, you’ll notice that the UBUNTU_VERSION environment variable is now included in the container’s environment:

$ docker compose up
[+] Running 1/0
 ✔ Container tmp-test-1  Recreated                                                                        0.1s
Attaching to test-1
test-1  | HOSTNAME=069e3c4a4413
test-1  | HOME=/root
test-1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
test-1  | UBUNTU_VERSION=24.04
test-1  | PWD=/
test-1 exited with code 0

This is incredibly convenient because maintaining the .env file allows you to easily manage environment variables across different services without modifying the compose.yaml file each time. This example clearly illustrates how powerful and useful it is to use .env files in Docker Compose configurations.