When a Docker Compose application has separate development and test configurations, repeating filenames, a project name, and port settings on every command gets tedious. A `.env` file in the current directory supplies default environment variables before Compose runs. section 10.3
Listing 10.6 An environment file for Docker Compose
# container configuration - ports to publish: TODO_WEB_PORT=8877 TODO_DB_PORT=5432 # compose configuration - files and project name: COMPOSE_PATH_SEPARATOR=; COMPOSE_FILE=docker-compose.yml;docker-compose-test.yml COMPOSE_PROJECT_NAME=todo-ch10
This `.env` file sets published-port variables and tells Compose which files and project name to use. Its `COMPOSE_FILE` value selects the core file plus the test override. section 10.3
Start the default test environment
From the configured application's directory, run `docker compose up -d`. With the shown `.env` file present, Compose uses the test override without additional `-f` or `-p` arguments. It creates both web and database containers, even though the core Compose file does not define the database service. section 10.3
A Compose file can also substitute these variables into service settings: `"${TODO_WEB_PORT}:8080"` uses the port supplied by `.env`. Do not confuse this automatic `.env` lookup with a service's `env_file:` entry, which names a file used for container application settings. section 10.3
Change the defaults to select another Compose configuration while keeping the same application image. Named application files such as `appsettings.Test.json` are a different approach: the application selects one through an environment name rather than Compose treating it as its `.env` file. section 10.5 section 18.2
