You will often see, when for example configuring providers, that the environment variables will look like this:

INSTAGRAM_CLIENT_ID=12345678901234567890

You have 2 options on how to set these variables in your docker-compose.yml file.

1. You can set them directly in the environment section of the service.

docker-compose.yml:

services:
  postiz:
    environment:
      YOUR_ENV_VAR: "value"
      YOUR_OTHER_ENV_VAR: "value"

2. You can use a .env file to set the variables.

docker-compose.yml:

services:
  postiz:
    env_file:
      - .env

.env:

YOUR_ENV_VAR=value
YOUR_OTHER_ENV_VAR=value

3. You can also use both!

docker-compose.yml:

services:
  postiz:
    environment:
      YOUR_ENV_VAR: "value"
    env_file:
      - .env

.env:

YOUR_OTHER_ENV_VAR=value

Now you need to note that when using an .env file, you will need to transder all enviroment variables from the docker-compose.yml file to the .env file. An .env file will ovveride any variables set in the .yml file. Using an .env file for the DB / Redis wont be neccessary.