To specify environment variables in a Docker Compose file, we use a key called
environment in the relevant service.
environment is an array, and to specify an environment variable, the format is simply
version: '3' services: backend: environment: - VARIABLE=VALUE
We can also specify just
VARIABLE, which will go through our shell for an environment variable that matches the name specified. It’s best to be explicit by going with the
VARIABLE=VALUE approach as our shell may not have the specified environment variable.